Continental Shelves during the Last Glacial Cycle: Knowledge and Applications
First IGCP 464 Annual Conference
Kong University, 25th-28th October 2001
|DAY ONE, Thursday 25th October, 2001|
0830 – 0900 Registration
0900 – 1015 Opening business meetin
SESSION 1 WESTERN PACIFIC SHELVES Chair: Donn Gorsline
1015 – 1040 Chemical evidence for marine/estuarine/lacustrine transistions in the Gulf of Carpentaria
- Allan Chivas, Matt Griffiths, David Wheeler et al.
1040 – 1110 Refreshment break
1110 – 1135 Growth history of coral reefs since the Last Glacial Maximum in the western margin of Australia ®
- Lindsay Collins
1135 – 1200 Stratigraphy and sea-level history of the late Pleistocene Sunda Shelf ®
- Till Hanebuth, Karl Statteger and Yoshiki Saito
1200 – 1225 Some main features of the bottom topography and the latest Pleistocene-Holocene sediments on the shelf of the Tonkin Gulf ®
- Tran Duc Thanh, Tran Dinh Lan, Dinh Van Huy et al.
1225 – 1250 Postglacial sea-level rise and palaeo-shoreline movement along the northern continental shelf of the South China Sea ®
- Y. Zong, Z. Huang and W. Zhang
1250 – 1400 Lunch break
SESSION 2 WESTERN PACIFIC SHELVES Chair: Nigel Ridley Thomas
1400 – 1425 Quaternary transgressive and regressive depositional sequences of East China Sea ®
- Zhenxia Liu, Ping Yin, Serge Berne et al.
1425 – 1450 Conceptual model of tidal sand ridge development since the last deglaciation in the continental shelves of Bohai, the Yellow Sea and the East China Sea ®
- Kelin Zhuang and Zhenxia Liu
1450 – 1525 Can the distribution of foraminifers in Holocene inner shelf sediment from the South China Sea be used as typhoon indicators? ®
- Guangqing Huang and Wyss Yim
1525 – 1550 Recognition of postglacial and pre-postglacial sediments on continental shelves: lessons learnt from the Hong Kong SAR, China ®
- Wyss Yim
1550 – 1620 Refreshment break
1620 – 1645 Distribution of diatoms in Holocene sediments in a core from Tai O Bay, Hong Kong SAR, China ®
- Michael Dickman, Wyss Yim, Guirong Wang et al.
1645 – 1710 Application of magnetic properties for studying modern seabed sediments contaminated by shipping activity in Hong Kong Harbour ®
- Lung Chan and Wyss Yim
1710 – 1740 General discussion
1800 – 1900 Reception
1900 – 2100 Buffet dinner Sweet 19 Restaurant, Graduate House
|DAY TWO Friday 26th October, 2001|
WHOLE DAY LANTAU ISLAND FIELD EXCURSION
0830 Sharp Depart from the Central Ferry Terminal for Mui Wo
0930 Chartered bus from Mui Wo to Po Lin Monastery to visit the Big Buddha
1115 Chartered bus from Po Lin to Tai O
1130 Study of sedimentation and palaeoenvironmental change in Tai O Bay
1300 – 1400 Lunch break
1400 Chartered bus from Tai O to Mui Wo
1445 Ferry to Penny’s Bay to visit the Disney theme park construction site
1700 Ferry to Tsing Yi via the sea channel between Ma Wan and Tsing Yi islands passing beneath the Tsing Ma Bridge and the Ting Kau Bridge.
1745 Train to Central
|DAY THREE Saturday 27th October, 2001|
SESSION 3 EUROPEAN SHELVES Chair: Lindsay Collins
0900 – 0925 Human activity of the Vistula delta plain and Vistula lagoon shoreline displacement during the Holocene ®
- Joanna Zachowicz
0925 – 0950 Relative sea-level curve of the southern Baltic ®
- Szymon Uscinowicz
0950 – 1015 Influence of the Holocene palaeoenvironment on shore protection measures in Flensburg Fjord, Baltic Sea ®
- Klaus Schwarze r
1015 – 1040 Continental shelf morphostratigraphic features due to last sea-level rise: certainties and uncertainties with examples from Mediterranean margins ®
- Francesco Chiocci
1040 – 1110 Refreshment break
1110 – 1135 Palaeoenvironmental analysis of submerged speleothems formed during the Last Glacial Maximum in Argentarola Island, Italy ®
- Fabrizio Antonioli and Sergio Silenzi
1135 – 1200 A high-resolution record of the late glacial maximum event in the western Black Sea ®
- Gilles Lericolais, Nicolas Panin, Francois Guichard et al.
SESSION 4 AMERICAN SHELVES & POSTER SESSION Co-chairs: Francesco Chiocci & Allan Chivas
1200 – 1225 Palaeogeography and early human adaption of the Queen Charlotte Islands, Canada: drowned landscapes, palaeo-coastlines, and palaeo-marine habitats ®
- Renee Hetherington, J.V. Barrie, R. MacLeod et al.
1225 – 1250 Post-LGM sedimentation on the outer shelf/upper slope of the northernmost part of the Sao Paulo Bight, southeastern Brazil ®
- Michel de Mahiques, Ilson Silveira, Silvia Sousa et al.
1250 – 1400 Lunch break
1400 – 1425 Geomorphological indicators of Quaternary sea levels on the continental shelf of southeastern Brazil
- Luis Conti and Valdenir Furtado
1425 – 1530 General discussion
1530 – 1600 Refreshment break
1600 – 1800 Poster session
1900 – 2130 Conference dinner
|DAY FOUR Sunday 28th October, 2001|
SESSION 5 SPECIAL Chair: Wyss Yim
0900 – 0940 Sea-level rise since the Last Glacial Maximum: the eastern Mediterranean Sea off Israel ®
- Gdaliahu Gvirtzman, Moshe Wieder and Nathan Bakler
0940 – 1030 Recent developments in offshore drilling technology on continental shelves*
- Ray Wood
1030 – 1100 Refreshment break
1100 – 1230 Business meeting
1230 – 1400 Farewell lunch
LIST OF POSTER PAPERS
1 Coccoliths and ostracods as indicators of the palaeoenvironment of the late Quaternary of the Gulf of Carpentaria (M.J.J. Couapel, J.M. Reeves, Allan Chivas et al.)*
2 Repetitive marine to lacustrine faunal changes in the Gulf of Carpentaria (Adriana Garcia, Allan Chivas, M.J.J. Couapel et al.)*
3 Palaeoenvironments of the Gulf of Carpentaria since the last glacial: reconstruction from palaeobiota (Adriana Garcia, Allan Chivas, J.M. Reeves et al.) ®
4 Mapping the seabed sediments of the southern China continental shelf and slope (Richard Hale)
5 Palaeogeography and early human adaption of the Queen Charlotte Islands, Canada: drowned landscapes, palaeo-coastlines, and palaeo-marine habitats (Renee Hetherington, J.V. Barrie, R. Reid) ®
6 Stratigraphy and sea-level history of the late Pleistocene Sunda Shelf (Till Hanebuth, Karl Statteger and Yoshiki Saito) ®
7 Can the distribution of foraminifers in Holocene inner shelf sediments from the South China Sea be used as typhoon indicators? (Guangqing Huang and Wyss Yim) ®
8 Palaeodeltas during the last glacial period in the outer shelf of the East China Sea (Shuanglin Li and Shaoquan Li) ®
9 Seismic and sedimentological characters of a 5th-order depositional sequence formed during the last glacio-eustatic cycle (E. Martorelli, Francesco Chiocci and G. Ercilla) ®
10 Palaeo-Indian archaeological evidence and two cases of land bridges in southern South America (Hugo Nami) ®
11 Clastic sedimentary facies of lowstand sea level during the Last Glacial Maximum in the continental shelf and shelf edge of the East Sea, southeastern Korea (Yong Park) ®
12 Palaeoenvironmental reconstruction of the Gulf of Carpentaria, Australia from the last interglacial to the present (J.M. Reeves, Allan Chivas, Adriana Garcia et al.)*
13 Influence of the Holocene palaeoenvironment on shore protection measures in Flensburg Fjord, Baltic Sea (Klaus Schwarzer) ®
14 Post-Last Glacial Maximum coastline change as a major forcing of regional hydrodynamic variations: an example from the eastern Brazilian continental margin (Silvia Sousa, Michel Mahiques, Raquel Passos et al.)
15 The offlap break position versus sea level: a discussion (Marcello Tropeano, Luis Pomar and Luisa Sabato) ®
16 The final stage of the Holocene transgression in the Puck Lagoon area, southern Baltic Sea as observed from the Rzucewo Headland case study (Szymon Uscinowicz and Grazyna Miotk-Szpiganowicz) ®
17 Submerged features related to the Last Glacial Maximum in the Argentine continental shelf: the present knowledge (Roberto Violante) ®
18 A preliminary study of the lower reaches of the Huanghe and Changjiang rivers during the Last Glacial Maximum (Dongxing Xia) ®
19 Holocene evolution of the Subei coastal plain, Jiangsu, China and the contributions of Changjiang and Huanghe sediments (Shou-Ye Yang, Cong-xian Li, Hoi-Soo Jung et al.) ®
20 Review of results of International Geological Correlation Programme project no. 396 ‘Continental shelves in the Quaternary’ (Wyss Yim) ®
* No abstract submitted.
Minutes and notes of the First Annual IGCP-464 Meeting, Hong Kong,25 and 28 October, 2001.
At the introduction on the opening day, the welcoming address was provided by Associate Professor Wyss Yim (Hong Kong SAR, China) who organised the conference. Professors Francesco Chiocci (Italy) and Allan Chivas (Australia) who had co-ordinated the proposal for the project, provided an outline of the scope of the project and a possible agenda for the later business meeting.
|F. Chiocci and
A. Chivas were elected co-leaders of the project.
advisorships to the project were accorded to Wyss Yim
(Hong Kong, SAR, China), Donn Gorsline (USA), Yong Park
(Republic of Korea) and Paolo Pirazzoli (France).
|The following national
representatives were elected: Roberto Violante
(Argentina), Lindsay Collins (Australia), Michel de Mahiques
(Brazil), Heiner Josenhans (Canada), Gilles Lericolais (France),
Klaus Schwarzer (Germany), Francesco Chiocci (Italy), Yoshiki
Saito (Japan), Yong Park (Republic of Korea), Szymon Uscinowicz
(Poland), Federico Vilas (Spain – as notified by email before the
meeting). The representative for China would be decided
upon and notified later.
National representatives were reminded that their responsibilities include collection and forwarding of their country’s project publications and the preparation of an annual report by early November in each of years 2 to 5 (i.e. 2002-2005).
were established in the following areas:
stratigraphy (leader: Gilles Lericolais, France)
WG-2 Chemical Stratigraphy (leader: Allan Chivas, Australia)
WG-3 Applied Aspects (i.e. economic deposits, engineering and geotechnical studies; leader: Wyss Yim, China)
WG-4 Influence on human culture (leader: Renée Hetherington, Canada).
This working group was added because of the relevance that the subjects offered by Nami, Hetherington, Lericolais, Zong, and others, have brought to the project. An effort will be made to involve archaeologists also in the next annual project meeting in Brazil.
|Offers had been received from
both New Zealand and Brazil to host the next annual project
meeting. After discussion, it was decided to award the
2002 meeting to Brazil, in part, because the IGCP-464 meeting
could be juxtaposed with the Brazilian Symposium on Oceanography,
in Sao Paulo, in late August, 2002, and which had an anticipated
attendance of 300 persons.
Immediately after the business meeting, an email was received from Tim Naish, from New Zealand, indicating that New Zealand supported the decision regarding Brazil, and would be pleased to offer to host the 2003 IGCP-464 meeting. Lindsay Collins had offered to host the 2003 meeting in Western Australia (Shark Bay, coral reefs) in case New Zealand were not available for that year.
(The arrangements for the Brazilian meeting were executed rapidly, with the first circular distributed on 18 December 2001).
It was considered useful to hold at least one annual regional meeting as well as formal global conferences. Accordingly, Lindsay Collins has convened a symposium on continental shelves, nested within the biennial Australian Geological Convention, Adelaide, 1-5 July, 2002. Szymon Uscinowicz offered to organise a meeting in Gdansk, Poland, in 2003, to summarise work from the Baltic region and possibly include the theme 'Last post-glacial sea-level rise and inundation of semi-enclosed basins.’
Marcello Tropeano discussed several field trips planned as part of IGC in Florence, 2004, that might be part of IGCP-464’s activities.
of medium and long-term goals of the project
|1. A major goal is the production
of reliable maps of coastlines at the LGM and other key
eustatic/climatic moments, that will take into account the
glacio/hydro-isostatic movements. This will be extremely valuable
because it will be based on true data, thereby groundtruthing
the models. To do that, a mid-term goal will be the definition
of all the possible proxies of sea-level position. A table will
be made so that any participant will suggest 1) palaeo-sealevel
indicators, and 2) the palaeo-sealevel depths in its own area.
|2. Another goal is the classification
of continental shelves, via the definition of end members,
and the establishment of a common terminology. To this
end, the following persons (some were absent from the meeting!)
were suggested as co-ordinators for particular regions for which
old and new basic data need assembly and synthesis in order
to classify shelves. These researchers and their regions are:
Collins and Chivas (Australia), Hanebuth (East Pacific), Voraya (India), Compton and Shaw (Africa), Chiocci and Gvirtzman (Mediterranean), Lericolais (North-east Atlantic), Polnyak (Arctic Eurasia), Josenhans (Arctic North America and north-west Atlantic), de Mahiques (Caribbean and South-east Atlantic), Gorsline (North-east Pacific), de Mahiques to contact Chilean colleagues (for south-east Pacific), Chivas and Violante (Antarctica). Allan Chivas agreed to collate some data that would assist with establishing criteria for classification. This would be greatly assisted by discussions and presentations to be held at the Australian Geological Convention in July 2002.
The website has been operational since mid-2001. Francesco Chiocci offered to install a capability for electronic discussion of relevant shelf-related topics on the website. Rather than encouraging idle chatter, it was suggested that discussion could be focussed under four themes:
(a) The relevant of isostasy in the definition of water palaeodepths reached at the LGM and other relevant eustatic markers.
(b) The definition of past coastline indicators and their relation with palaeodepths.
(c) The definition of the contribution that the study of shelves may give to the carbon-cycle project.
(d) The compilation of the description and classification of continental shelves.
Luis Conti offered help in setting up these aspects of the web page and in electronically formatting the posters presented at the conference.
|(a) Gilles Lericolais suggested
that proposals seeking financial support to operate the
project’s training workshops, might be addressed to the
IGCP office in Paris.
|(b) Several participants drew
attention to the general lack of numerical modellers within
the project’s registrants. It was acknowledged that our
members would need to work closely with, and follow the
work of, researchers who are modelling the dynamics of past
and present ice-sheets, sea-level changes and isostasy.
CONFERENCE ABSTRACTS (click on read dot to read the abstract)
The Last Glacial Maximum in La Plata River, Argentina
José Luis Cavallotto - Division of Marine Geology and Geophysics, Argentine Navy Hydrographic Office, Buenos Aires, Argentina
The geological evolution of the Argentine continental shelf during the late Cenozoic was related to the occurrence of alternating transgressive-regressive events associated with glacial-eustatic processes. The best preserved geological record corresponds to the late event of sea-level fall and rise which includes the LGM.
La Plata River constitutes today a fluvial-estuarine system where fluvial and tidal dynamics actively interact. The river and surroundings comprise one of the areas of the entire Argentine littoral region where most significative geomorphological changes occurred around the LGM due to interaction among sea-level changes, fluvial activity, estuarine dynamics, coastal processes and high sedimentation rates.
Prior to the LGM, during the final stages of late Pleistocene sea-level fall, continental reliefs (today located on the continental shelf) were reactivated by erosive processes, which were highly significative in fluvial valleys. As a result, the ancient La Plata river palaeovalley was deeply excavated and hence erosive features dominated there without representative depositional features.
The most important positive geomorphological feature in this part of the Argentine coasts was at that time the emerged plain which behaved as a headland separating the La Plata river paleovalley from the Salado river valley located to the south. During the sea level fall, regressive littoral environments such as barrier islands, beach ridges, lagoons and tidal flats formed at the sides of the headland exposed to the sea and prograded seaward before reaching its outer edge. Relicts of these features are best preserved in the flattest areas located further south of the headland (Violante et al., 2001). The regressive deposits, today preserved under the shelf surface, constitute a falling stage system tract.
At the LGM, the sea-level was below ca. 150 m at 16,690 yr BP. (Guilderson et al., 2000). At that time, the river flowed along a narrow valley which ran along the present Uruguay and Rio Grande do Sul coasts. Through this valley, the waters coming from the tropical regions of northeastern Argentina, Brazil and Paraguay drained into the sea, carrying away a considerable amount of sediments which were deposited at its mouth as imbricate cones thus giving origin to a deltaic body, which includes prodelta facies located even seaward (Urien and Ewing, 1974).
The presence of these sediments representing low sea-level deposits, offlaping structures, relict paleovalleys and unconformities, as they are observed in seismic records, allows to define in the sequence stratigraphical record a lowstand system tract.
With the beginning of the postglacial sea-level rise (Holocene transgression), when the sea invaded the river valley, the littoral sandy sediments at its mouth were reworked and an estuarine environment was formed and infilled with muddy sediments resulting from processes of clay flocculation in the maximum saline gradient zone (MSGZ), giving origin to a muddy depocenter. As sea-level rose, the depocenter moved upslope as MSGZ migrated inland and filled up the former paleovalley, at the same time that fluvial supply decreased as base-level went up.
Application of magnetic properties for studying modern
seabed sediments contaminated by shipping activity in Hong Kong
L.S. Chan and W.W.-S. Yim - Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
Measurments of magnetic susceptibility and heavy-metal concentrations of continuous vibrocore samples were found to be indicators of shipping contamination in seabed sediments in Hong Kong Harbour. A significant correlation was found to exist between magnetic susceptibility and the concentrations of Pb, Zn and Cu in the upper 1 to 2 m of the cores. This is attributed mainly to paint-coated rust fragments containing red lead broken off from the body of ships and released into the seabed sediments either through wear and tear or during ship repainting when the old paintwork is removed by scraping. The contamination of modern seabed sediments by ships is exacerbated by Hong Kong being a leading shipping and container port in the world.
Chan, L.S., Ng, S.L., Davis, A.M. et al. (2001). Magnetic properties and heavy-metal contents of contaminated seabed sediments of Penny’s Bay, Hong Kong. Marine Pollution Bulletin 42/7: 569-583.
Continental shelf morphostratigraphic features due to last sea-level rise: certainties and uncertainties with examples from Mediterranean margins
Francesco L. Chiocci - Department of Earth Sciences, University of Rome “La Sapienza”, P. le Aldo Moro 5, 00185, Rome, Italy
The last glacio-eustatic hemi-cycle (from the last lowstand to the present-day highstand) deeply modified continental shelves at a global scale. Almost all the shelf characters (morphology, sedimentology, shallow stratigraphy) witnessed the dramatic environmental changes tied to a sea level rise of more than 100 m in a quite short time (some 10,000 years). As a consequence, all over the world the continental shelves exhibit similar transgressive features. The main characters of the Mediterranean shelves (microtidal, medium energy, siliciclastic) will be described and discussed.
The shelves stratigraphy is truncated by an erosional unconformity that was caused by shelf emersion during last glacial era. In this period river valleys scoured the shelf and gave rise to incised valleys if front of river mouths. Above the erosional surface seldom transgressive relict sand bodies are found, forming narrow belts parallel to the coast, with relief of some metres with respect to the surrounding seafloor. They formed during sea level rise at or near the retreating shoreline. They are rare and poorly developed because, due to the fast sea level rise, the transgression was usually non-depositional, i.e. the littoral wedge migrated landwards without leaving any deposits beyond but for a thin shell and pebbly lag. Where transgressive deposits are thicker, they are made up of littoral sand with an inner prograding structure. Above the transgressive deposits (where present) or resting directly on the erosional surface, the present-day highstand sedimentary wedge develops. In microtidal shelves it is made up of littoral sand down to a depth of 15-20 m; at greater depth it is made up of shelf mud, as thick as a maximum of several tens of metres in front of the main river mouths. The highstand wedge formed since 8,000 yr BP, when sea level reached its present position; between 8,000 and 6,000 yr BP subaerial basins were filled and a great quantity of sediment was able to reach the coast, widening the coastal plain especially in deltaic areas in the last thousands of years. Where Holocene mud is absent, lowstand or transgressive relict sediments crop out on the shelf floor.
With respect to the above described situation, that can be considered as a standard for intermediate latitudes, some transgressive features are not fully explained by comparison with present-day processes or by the model we assume to work in the “geologic world”.
OPEN QUESTION 1: RAVINEMENT SURFACE
The erosional unconformity truncating the shelf stratigraphy is polygenic, as it was created in each single point of the shelf by 1) shoreline erosion during sea level fall; 2) subaerial erosion during shelf exposure at lowstand; 3) shoreline erosion during sea level rise (ravinement process). Because of its flatness and absolute lack of any morphological irregularity, subaerial erosion is likely to have been the less effective factor in reshaping shelf topography (despite the fact that it is the more cited in literature and used in models). Erosion during sea level fall is probably responsible of the formation of the prograding structure of the shelf with the emplacement of “forced regression” wedges on the continental slope. Erosion during sea level rise (ravinement) is likely to have occurred, but the process is not easy to deduce. In fact the usual lack of transgressive deposits witness a non-depositional transgression with a shoreface retreat mechanism. However, if coastlines migrated landwards, eroding substratum without leaving any sediment beyond, the products of the ravinement would have been carried across the shelf increasing in volume as sea level rise proceeded. A rough estimate of just a few metres of erosion would account for an enormous amount of sediment that could have been accumulated at the eustatic maximum. Evidence of such deposits is commonly lacking.
OPEN QUESTION 2: INCISED VALLEYS.
During last glacial time, sea level was lower than present and river valleys scoured the shelf, digging incised valleys. Both the oxygen-isotope curve and evidence of erosion, point out a minimum sea level at or near the shelf break (some 110-120 m water depth). Incised river valleys should therefore extend down there. Surprisingly, on the contrary they usually did not extend any deeper than about 70 m. Moreover they are deeper in the inner shelf whereas in the middle shelf they loose morphologic expression. A geologic/palaeohydrologic model is needed to explain such evidence such as: 1) erosion during sea level rise (why mainly in the outer shelf?) ; 2) very different palaeoclimate conditions (arid climate during glacial, humid in the interglacial) with creation of incised valleys during sea level rise instead of during sea level fall or lowstand (very different idea with respect to literature model); 3) linear/braided transition of the fluvial network from inner to outer shelf (why?).
OPEN QUESTION 3: TRANSGRESSIVE DEPOSITS
Even if the transgression was usually non-depositional, i.e. left over only a thin lag, somewhere on the shelf transgressive deposits are found. They are made up of littoral fine sand, have a inner prograding structure, a flat base and a convex top. They are arranged in belts some hundreds to a thousands of metres wide, elongated parallel to the isobaths for several km or tens of km. They are not present all along the margin (as is the case of lowstand deposits) neither offshore the main sedimentary sources (as is the case of the highstand deposits). Rather they are found close to morphologic highs, representing palaeo-headlands during transgression. Therefore a specific littoral process is needed, different from present deltaic or strand plain model, with a more efficient along-shore transport and/or a diffuse feeding of the shelf from the coast rather than a feeding from a few point sources.
Growth history of coral reefs since the Last Glacial Maximum
in the western margin of Australia
Lindsay B. Collins - Department of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia
The western continental margin of Australia is bordered by carbonate shelves which extend from temperate to tropical latitudes. Coral reef systems are discontinuously developed, and record a history of Quaternary sea level change. The reefs vary from shelf-edge, atoll-like systems, to fringing reefs, to isolated platforms akin to downslope buildups found in the geological record. Their Late Quaternary evolution is being documented using a combination of U-series dating and seismic methods.
The Abrolhos carbonate platforms, at 28-29.58S are shelf-edge reefs which have provided a detailed record of Last Interglacial and Holocene sea level change, which is a reference for the central part of the margin, and is comparable to global records of sea level change. The Ningaloo fringing reef, at 20-228S, records Holocene and Last Interglacial reef growth which is less complete than the Abrolhos record.
Recent research has concentrated on Scott Reef and the Rowley Shoals, which rise from depths of 500 m and lie to seaward of the Northwest Shelf. Scott Reef surfaces a carbonate platform which is a major oil and gas reservoir. The reef consists of the nearly closed North Reef and the boomerang shaped South Reef separated by a 400 m deep intervening trench. The 45 m deep lagoon of South Reef has healthy coral cover and hard substrate sponge-algal-coral communities, but is starved of sediment. Strong tidal flushing (tidal range is 4 m) and the open nature of the lagoon are important controls. Seismic profiles reveal an earlier stage of growth occurred 125,000 years ago, but reefs which apparently reached sea level are 30 m below present sea level and the lagoon floor is 50 m below sea level. Up to 30 m of subsidence may have occurred in the last 125,000 years. The older reef system was colonised by Holocene reefs about 10,000 years ago, which probably grew to sea level by 5,000 years ago and comprise the modern "double" reef crest. This Holocene reef is up to 35 m thick.
The Rowley Shoals comprise one of the most perfect morphological series of reefs known. These emergent, annular reefs rise from the Scott Reef/Rowley Shoals Platform, between the 300 m and 700 m contours. Seismic profiles suggest a similar reef growth and subsidence history to that of Scott Reef applies to the Rowley Shoals, but reef architecture differs between the two systems investigated to date.
Both the Rowley Shoals and Scott Reef are under increasing pressure from fisheries, tourism and illegal immigration, and the information provided by studies of growth history will be of use in developing management plans for the reefs.
Distribution of diatoms in Holocene sediments in a core
from Tai O Bay, Hong Kong SAR, China
Michael D. Dickman1, Wyss W.-S. Yim1, Guirong Wang2 and Guangqing Huang3 1 Department of Earth Sciences, The University of Hong Kong, 2 Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 3 Guangdong Institute of Geography,
A 14-m Holocene section of a vibrocore from Tai O Bay in Hong Kong was studied for its diatom distribution and sedimentological properties including sediment type, magnetic susceptibility and radiocarbon age. The diatom assemblage present in the core was indicative of a tropical to subtropical environment. Thalassionema nitzschioides, a pinnate diatom present, is indicative of elevated nutrient levels. This diatom was only common in the top 0.1 to 0.35 m of the core. Two sections of the core displayed very low diatom concentrations. Firstly, in the top 0.6 m of the core, the low diatom concentration was attributed to the increase in energy level associated with tidal and wave action. Secondly, at a depth of 9.6 to10 m, the low diatom concentration was interpreted to represent a lower sea-level stillstand about 10 m below the present day also accounted for by the increase in energy level associated with tidal and wave action which prevented the deposition of diatom frustules.
Freshwater diatoms such as Fragilaria spp. were only occasionally found and their valves were commonly broken suggesting an allochthonous contribution. The relative absence of Fragilaria spp. and the dominance of Paralia sulcata throughout the core indicated that the Tai O site was occupied by marine waters with only minor influences of freshwater throughout much of the Holocene at least until recently. This appears to be in agreement with the greater influence of the discharges from the Pearl River Delta in the present day.
The influence of palaeogeography on human development in
the southern Po Plain during the last 1 Ma
Farabegoli Enzo, Onorevoli Giuseppe and Bacchiocchi Camilla - Dipartimento di Scienze della Terra e Geologico-Ambientali, Via Zamboni 67, 40126 Bologna, Italy
A strong tectonic uplift caused the first nearly-generalized emersion of the apenninic chain between 1.35 and 1.15 Ma. The paleo-Apennines mountain belt faced directly (i.e. without a surrounding alluvial plain belt) the paleo-Adriatic Sea up to 1,000 m depth. The progradational shallow marine units (Imola Yellow Sands fm.) correspond to the Matuyama-Jaramillo paleomagnetic transition (about 0.99 Ma). The marine deposits culminated as delta-fan and continental regressive facies covered everywhere by continental sediments (paleo-Po Plain) rich in decimetric-sized gravels too, referable to the Matuyama/Bruhnes transition at 0.78 Ma. The infilling architecture of the padan basin was characterized by aggradation of continental sediments repeatedly interrupted by warm-age transgressive/prograding sub-tabular shore sediments. The latest warm episod corresponds to the well mapped Flandrian ("Versilian") transgression.
The inner margin of the southern Po Plain was occupied by the first human groups from the end of the Lower Pleistocene, about 760,000 years ago. An assessment of the data available for the Italian peninsula as a whole, shows the presence of a clear interruption separating these first traces of occupation from the subsequent development of industries throughout the territory in the early Middle Pleistocene. The Lower and Middle Palaeolithic period has hence been subdivided into five cultural stages, the lithic industries belonging to each phase being characterised by different technological features procedures and knapping techniques: pebble industries, Clactonian and Protolevallois industries, Acheulean industries, Levallois industries with bifaces, and Middle Palaeolithic industries.The majority of lithic finds along the Emilia-Romagna apenninic margin was associated with high-stand sediments (warm climate), with the exception of Levallois industries, often found into löess deposits of the closing stage of the Riss glacial.
The shoreline shifted by about 5 km during each high-stand warm period and at the end of Paleolithic the Po Plain enlarged to 10-40 km whereas the North Adriatic Sea shallowed to less than 200 m. With the end of the Palaeolithic and the appearance of the first groups of hunter-gatherers of the Mesolithic period, there was a substantial change in the settlement patterns: the groups began to develop economies based on exploitation of various types of ecosystems, also on a seasonal basis, from the marshy areas in the plain to the mountainous pass areas along the Apennines chain. The rogression in the Mesolithic industries can hence be followed by exploring a series of sites situated not only near the main streams, but also within the complex network formed by secondary watercourses.
At the start of the Neolithic, with the settlement of human groups, the gradual occupation of the plain became more and more widespread, reaching completion during Roman times; the territory was now subjected to extensive operations of deforestation, division into holdings and estates, and cultivation on a large scale. Severe subsidence led to the rapid burial of the ground surfaces and artefacts of the Po Plain.
The regressive north-eastern shifting of the coast line produced alluvial plain and channels almost without gravel. Consequently it become impossible to find pebbles in progressively wider and wider (10-40 km in north-south extention) hunting suitable areas. The improvement of manufacturing techniques produced lighter, more useful and durable lithic instruments. This evolution could be interpreted as the local adaptative response to carrying these instruments for longer periods and distances. Moreover, cultural changes due to external migrations don't conflict with this hypothesis.
The termination of the Würm glacial period, about 15 ka ago, is marked by the very rapid raising of the sea level, known as the world-wide Flandrian transgression. The extremely flat bottom of the Adriatic area favoured the quick northwestwards migration of the coastline, at a medium rate of about 300 m/yr along the basin's axis. The transgression was so fast that for most part of the migration, the Po river mouth left no valuable sedimentary records. Prograding litoral sandy dunes (1-3 m thick), covered by thin lagoonal peaty sediments, emerged only during limited time lapses characterized by the slowing down of the rate of raising or the eustatic uplift. A group of 9-7.5 ka B.P. old prograding littoral sandy dunes were found at a depth ranging from -40 and -30 m in the northern Adriatic Sea.
About 5.5 ka B.P. the maximum flooding line of the Flandrian transgression was 2-15 km up the present coastline north of Cervia, and more than 50 km up the Po delta-front. Later, the strong yield of fluvial sediment filled and eventually by-passed the peri-apenninic depocenters. The filling was a two-phase process: at first the build-up of a high subaerial dune barrier which divided an internal lagoon from the sea and then, the partial filling of the lagoon leaving a fresh or brackish-water marsh with the accretion of a new external sandy barrier. Careful geomorphologic-historical researches and a remarkable number of archaeological sites have allowed the accurate definition of aggradation and progradation sedimentary processes since the Bronze age. Good examples of the huge filling and sedimentation rates are the well studied roman-age country house sited at a depth of 10 m about 15 km from the present coastline and the Classe harbour sited at a depth of 5 m about 6 km from the present coastline. Locally, medieval remains lie deeper into the more subsident area around Ravenna.
Regarding the progradation rate of the shore line, we can cite data measured along a section WSW-ENE oriented, located 25 km northward of Ravenna: the range 5.5-4.0 ka B.P. is characterized by a very high accretion rate: 10 m/yr, slowing down to 4.2 m/yr in the interval 4.0-2.8 kyr B.P and then to 2.8 m/yr in the span between 2.8 and 1.9 kyr B.P. The accretion rate increase to 4.2 m/yr in the interval 1.9-0.7 kyr B.P and to about 5 m/yr between 0.7 kyr and the present. The Po delta advanced over 15 km between 5.5-2.0 kyr B.P, and about 30 km during the last 2 kyr. Since the Appennine belt was the main source of terrigeneous sediments of Po and the others apenninic rivers, it’s obvious to correlate the increase of erosion and solid transport rates to the progressive decay of mechanical properties of soils and bedrock due to expansion of agricultural activities on hill areas. The post-roman age high erosion rate of the Apennines, can be explained by the rapid depletion of the forest cover, a process which began in the etrurian (iron) age, and consistently widened through roman times, the Renaissance and the industrial era.
The use of finite-element software that simulates the filling of sedimentary basin allowed us to study the setup of flandrian wedge between Cervia and the Po River. We adopted as eustatic reference the Fairbrdge curve, calibrated with local historical data to evaluate the volume of the depositional system (coastal sand / alluvial plain) at about 56 x 109 m3. The high sediment flow of some apennine catchment basins (evaluated erosion rate of 0.5 - 1 mm/yr) counterbalanced the long-term strong tectonic subsidence (1-1.5 m/kyr) in the coastal area between Cervia and the Po River. The progradational trend ceased in the late sixties, due to sudden reduction of sediment flow and sediments compaction (in consequence of strong aquifers sinking) triggering subsidence and coastal erosion processes. At present, many areas are the center of relevant tourist, industrial and farming activities (budget > one thousand million dollars/yr), lies 1-2 m below m.s.l and require water-scooping machines to manage. Holding present sedimentary and hydrogeologic parameters, our simulations show that the coast line could shift inside the southern Po Plain from a minimum of 1 to more than 6 km. A recent proposal to reduce consequences of coastal erosion consists of to draw out flandrian sands (200 x 106 m3) from central adriatic basin on the shore. The modeling results point out that solution can be effective only for the limited span of some tens of years. Only the rapid restoration of historical parameters will allow the preservation of the alluvial plain and coastal environments for the next century.
Holocene evolution of the Sixteen Mile Beach complex,West
Coast National Park, South Africa
Franceschini, G. and Compton, J.S. - Department of Geological Sciences, University of Cape Town, South Africa
The Sixteen Mile Beach complex is located 100 km north of Cape Town and it is formed from long-term accumulation of sand transported by longshore drift along the inner shelf northward. The complex is composed of three distinct units: Sixteen Mile Beach, a 26 km long log-spiral beach of varying width; shore-parallel coastal dunes and, in the southern part, a 24 km long inland sand plume with mobile and immobile dunes.
Grain-size analyses have shown that Sixteen Mile Beach is composed of predominantly fine sand from the southern end to 6 km northward. Between 6 and 9 km north of the southern end there is a rapid decrease in the fine sand fraction and an increase in the amount of medium sand. >From 12 km northward the beach is made up predominantly of medium sand with a minor coarse sand fraction that increases northward. Offshore bathymetry influences the distribution of the sand along the beach. Dassen Island in the southern part of Sixteen Mile Beach, creates a protective environment where the fine sand is focused. The beach sand is primarily composed of quartz and shell fragments. The CaCO3 content of the beaches ranges from 41 to 55 %.
Coast-parallel dunes are made up of a mixture of fine and medium sand, with an increase in the fine sand downwind. These dunes consists of a non-vegetated, wind deflated shell layers composed largely of Donax serra and Choromytilus meridionalis, with terrestrial dunes composed of quartz and shell fragments (CaCO3 ranges from 35 and 65 wt %). The dune plume is composed of very well sorted fine sand both in the mobile and immobile dunes. The CaCO3 content of the active dune plume decreases from 45 % to 3 % at 24 km inland.
Radiocarbon dates were obtained for 41 samples of both whole shells and sand-sized shell fragments within the complex. Along Sixteen Mile Beach the bulk carbonate sand fraction has calibrated radiocarbon ages that ranges from 18,700 in the southern part to 10,800 years in the northern area. Younging of the beach to the north reflects the input of coarser shell material from the northern rocky headlands. The active dune plume is 4,500 years old at 24 km inland suggesting that the dune plume was initiated after the sea-level returned to its present-day position. Different rates of sand movement along the active plume are indicated by radiocarbon results. In the coast-parallel dunes age range from 900 years to 3,200 years in the active foredunes while the stable vegetated dunes are 4,800 years to 8,000 years. Radiocarbon dating on many articulated Patella shells from middens indicate human utilisation of the area by 6,200 years, significantly earlier that other sites in the Langebaan Lagoon area.
Foraminiferal assemblages of the Langebaan Lagoon salt
marsh, West Coast National Park, South Africa
G. Franceschini1 and I. McMillan2 - 1Department of Geological Sciences, University of Cape Town, South Africa - 2 DeBeers Marine Limited, Cape Town, South Africa
Foraminiferal assemblages have been used to determine the effect of sea-level change on different areas in the Langebaan Lagoon, on the southwest coast of South Africa and successively have been used to detect past lagoonal environments along the west coast of South Africa. Salt marsh foraminifera are among the most valuable group of sea-level indicators as their distribution shows a narrow vertical zonation which can be accurately related to sea-level change, salinity and vegetation cover.
A transect was sampled along the southwestern edge of the lagoon from tidal flat to high marsh. Above a Zostera capensis muddy quartzose sand tidal flat there is low marsh consisting of pioneer Spartina maritima, and a narrow strip with Chenolea diffusa and Salicornia meyeriana. Beyond the low marsh there is an upland community with a vegetation cover of 100 % not directly influenced by tidal water. Vegetation cover of Chenolea diffusa, Salicornia meyeriana, Trigloshin bulbosa, Pulcinella spp, and Sarcornia pillansii characterize the middle marsh. Chenolea diffusa and Salicornia meyeriana dominate the high marsh. The tidal flat is a mixture of sand (70 %) and silt. From the low marsh to high marsh, the sand fraction decreases and is replaced by increasing silt and clay fractions.
The high supratidal salt marsh is dominated by the agglutinated foraminifera Trochammina inflata, the middle salt marsh consists of a mixture of other Trochammina inflata and Jadammina macrescens and the intertidal zone is dominated by the calcareous species Ammonia japonica, Ammonia parkinsoniana, Elphidium articulatum, Elphidium sp. A and Quinqueloculina sp.
The foraminiferal zonation shows a strong relationship to elevation, which has important implications for establishing records of relative sea-level change. The salt marshes in Langebaan Lagoon, can be divided into three zones based on different foraminifera assemblages. Zone I is dominated by calcareous species, Zone II by agglutinated foraminifera, Zone III by a monospecific assemblage.
Palaeoenvironments of the Gulf of Carpentaria since the
last glacial:reconstruction from palaeobiota
A. Garcia1, A.R. Chivas1, J.M. Reeves,1 M.J.J. Couapel1, S. Van Der Kaars2, S. Holt1 and P. De Deckker3 - 1School of Geosciences, University of Wollongong, - 2School of Geography and Environmental Sciences, Monash University, Clayton, - 3Department of Geology, The Australian National University, Canberra, Australia
This is a preliminary palaeoenvironmental reconstruction of the Gulf of Carpentaria, Australia, since the Last Interglacial based on assemblages analysis of Foraminifera, Charophyta, Ostracoda, nannofossils and pollen. The multidisciplinary work was performed on the longest of the six cores (14.8 m long) collected in 1997 in the Gulf.
At 14.8 to14 m there is a sandy layer, oxidised, indicating a non-marine, fluvial environment. The transgression occurs around 14 m, and is represented by marine organisms. The marine biota indicates a fluctuating marine environment with an alternation of shallow/deeper conditions up to 9.5 m. At this depth the associations change to non-marine up to 7.1 m, with some marine incursions intercalated. Between 7.1 m and 5.8 m depth, littoral fauna plus concretions indicate a mix of exposure and marine incursions. At 5.8 to 5.6 m depth the sediments are mainly quartz-rich, sterile, and comparable with a delta type of deposition. Foraminifera, coccoliths and ostracods are present in these levels, indicating the various changes.
The interval from 5.6 m up to 0.4 m is mainly non-marine, showing some minor marine incursions up to 3.6 m. From 3.6 to 0.4 m, Lake Carpentaria shows a progressive increase of fresher water biota. Between 0.9 m to 0.5 m the lake is fresh, with abundant species of charophytes and ostracods. The final marine transition occurs at ca. 10 ka BP, at 0.4 m, after which the fauna is fully marine up to the recent sediments of the core.
Dates obtained by TL and OSL gave an age of ca, 125 ka BP for the sandy layer at the base of the core. Radiocarbon ages indicate the final transition occurred at ca. 10 ka BP, with the time of maximum freshwater input in the lake occurring at ca. 12 ka BP.
Further studies will involve a more exhaustive analysis of the assemblages, the geochemistry of selected taxa, and new dates, to provide more accurate information about the history of the gulf.
Sea-level rise since the Last Glacial Maximum: the eastern
Mediterranean Sea off Israel
Gdaliahu Gvirtzman1, Moshe Wieder1 and Nathan Bakler2 - 1 Department of Geography, Bar Ilan University, Ramat Gan, 52900, Israel - 2 Geological Survey of Israel, Jerusalem, 524 91, Israel
Alternations of marine and continental formations are very common in boreholes and outcrops, in the Quaternary sequence of the coastal belt of Israel, onshore and offshore. In a borehole, Ashdod 321, located some 20 km offshore, central Israel, next to the Ashdod harbour, a sequence of these alternations was penetrated. The 77 m of a continuous section is composed of six stratigraphic units, below sea bottom, numbered from bottom to top, as follows (thickness of unit and water depth of each unit, see figure below):
(6) 9 m (0 to 9m) of sea water from sea level to seabed.
(5) 16 m (-9 to 25 m) of marine foraminiferal loose sand. (The uppermost unit)
(4) 15 m (-25 to -40 m) of a genuine dark gray Grumusol soil type (Vertisol), with abundant carbonate white nodules at the base, similar to those occurring in loessial soils, in the desert fringes of the Middle East (Ya'alon and Ganor, 1979). The calibrated age by C-14 of charcoal particles, found within the upper part of the soil, is 10,450 yr BP.
(3) 13 m (-40 to -53 m) of marine calcareous, quartzic, foraminiferal loose sand.
(2) 12 m (-50 to 62 m) of Grumusolic soil material, and some swamp layers and black peat. The calibrated ages of by C-14 of charcoal particles from the lower part of the black peat is 39,750 + 570 yr BP and from the upper part of the black peat is 37,750 + 570 yr BP.
(1) 15 m (-62 to -77 m ) (total depth of the borehole) of sandy eolianites, cemented continental dune sand (the lowermost unit).
In outcrops exposed in the coastal cliff, along the seashore of central Israel, composed of alternating eolianites, coastal sand dunes and red Mediterranean sandy soils (Rhodoxeralfs) are very common. Near the top of the section a unit of 0.5 to1 m dark-gray Grumusol or clayey material is found intercalated between units of red sandy soils. At the lower part of the Grumusol some carbonate nodules occur. The ISLR (luminescence) age of the lower red soil is from 40,000 to 12,500 yr BP; the Grumusol is from 12,500 to 11,500 yr BP; the upper red sandy soil is from 10,000 to 7,500 yr Bp (Gvirtzman and Wieder, 2001). The micromorphology of the exposed Grumusol indicates that the origin of the soil material is eolian dust. The red soils were formed under wet - rainy environment of a Mediterranean climate (Ya'alon and Ganor, 1979). The Grumusol unit correlates, in facies and in age, to the Grumusol (unit 4) in the offshore Ashdod 321 corehole. The magnetic susceptibility (Mullins, 1997) of the Grumusols in the outcrop cliff and in the borehole is (in units of : m3 * Kg -1 * ( -8 10 5 to 20 susceptibility units. The red soil formations are of 25 to 75 susceptibility units, (Gvirtzman and Wieder, 2001; see also Kukla et al., 1988).
The micromorphology of the soil units and the lithology of the whole sequence show a clear distinction between the marine and the non-marine units, but the patterns of the two types are similar. There is a very clear unconformity surface at the top of Unit 3, which is the bottom of Unit 4. Unit 4, the Grumusol sequence, is therefore, the most important and outstanding feature. The matrix of the two Grumusol units (found in the onshore and in the offshore sections) is a typical desert loess material, in which dust of dry to semi-dry desert origin was accumulated (Goudie, 1978). Soon after, the dust was washed intensively by rain-water. The carbonate was leached and accumulated as a secondary carbonate at the depth of rain-water penetration. The common feature, at present in modern environments, is the down -leaching of the carbonate particles from the dust to the bottom of the loess soils and the accumulation of carbonate nodules at the bottom (Wieder and Gvirtzman, 1999).
The soil unit within the studied marine sequence is related to the global history of sea levels. This issue is discussed below. The two red sandy soils that envelope the Grumusol in the outcrops of the coastal cliff are correlated to the two wet events, before and after the Younger Dryas Event, designated in the Mediterranean as "MWP 1A" and "MWP 1B". The wet phase after the dry event fits the accumulation of "Sapropel S1" that was deposited on the floor of the Mediterranean (Fontugne et al., 1994; Troelstra and van-Hinte, 1995). This process is well noted in the southern belt of the Mediterranean Climate. This feature was recognized in modern environments in the Negev Region of Israel and in North Africa.
These observations are correlated by us to a global pause in the rise of the sea level. The global age of the well-known Younger Dryas Event is ca. 11,0000 to 10,000 yr BP. This age fits the dated Grumusols. The global pause of the rise of sea level was associated with climate changes. A global cooling and accumulations of an extended ice cap were reported. Desert dust accumulations around the global deserts were recorded; a significance drop of sea level was associated with the renewal of the polar ice caps. The widespread exposure of dry continental shelves was associated with increased area of the continents and of human habitation on the dry exposed areas (Berger and Lebeyrie,1987).
The information from the eastern Mediterranean is a new additional measure of the scenario of the global rise of sea level (relative to the present sea level). From the global glacial maximum (at a minimum elevation of -130 m) a global rise took place. We found that approximately at about -25 m water depth, the beginning of the first pause took place (at about 12,000 yr BP), within the rising scenario. At about water depth of -42 m (equals the maximum event of Younger Dryas) the pause stopped (at about 11,000 yr BP). From the end of this pause, the new continuous modern rise took place up to the present. We correlate the pause that we found in the eastern Mediterranean to the global event of the Younger Dryas. The vertical drop in sea level during theYounger Dryas Event, according to our measure, is approximately 17 m (from –25 to –42 m). The water volume of this drop, in a global aspect, equals approximately the additional ice volume that was accumulated as ice in the polar regions. This new observation, from our semi-desert region of the Eastern Mediterranean, indicates the global nature and the widespread distribution of the Younger Dryas event. The exact age of the event can also be deduced from radiocarbon dating of charcoal particles in the Grumusols of this event. The local terminology used in our region in local publications is: "Kurkar" = "Eolianite" = indurated and cemented eolian dune sand (i.e. unit 1 as in the Ashdod 321 borehole); "Hamra" soil = red Mediterranean sandy soil = Rhodoxeralf (i.e. found in outcrops of the coastal cliff, the two units that envelope the grumusol).
Berger, W.H. and Labeyrie, L. D. (1987). Abrupt climatic change - an introduction. NATO ASI SERIES, Reidel, 3-22.
Fontugne, M., Arnold, M., Labeyrie, L., Paterne, M., Calvert, S. E. and Duplessy, J. C. (1994). Palaeoenvironments, sapropel chronology and Nile River discharge during the last 20,000 years as indicated by deep-sea records in the eastern Mediterranean. In: Bar-Yosef and Kra (eds.), Late Quarternary Chronology and Palaeoclimate of the Eastern Mediterranean. Radiocarbon, pp. 75-88.
Gvirtzman, G. and Wieder, M. (2001). Climate of the last 53,000 years in the eastern Mediterranean, based on soil-sequence stratigraphy in the Coastal Plain of Israel. Quat. Research (in press).
Goudie, A.S. (1978). Dust storms and their geomorphological implication. J. of Arid Environment 1: 291-319.
Kukla, G. and An, Z. (1989). Loess stratigraphy in central China. Palaeogeography Palaeoclimatology Palaeoecology 72: 203-225.
Mullins, C.E. (1977). Magnetic susceptibility of soil and its significance in soil science - a review. Soil Science 28: 223-246.
Troelsma, S.R. and van-Hinte, J.E. (1995). The Younger Dryas - Sapropel S1 connection in the Mediterranean Sea. Geology Mijnbow 74: 275-280.
Wieder, M. and Gvirtzman, G. (1999). Micromorphological indications on the Late Quaternary in the southern coastal plain of Israel. Catena 35: 219-237.
Ya'alon, D.H. and Ganor, X. (1979). East Mediterranean trajectories of dust - carrying storms from the Sahara and Sinai, In: Morales, C. (ed.), Saharan Dust Mobilization, Transport, Deposition. Wiley, New York, pp. 189-193.
Mapping the soils of the southern China continental shelf and slope
Richard Hale - EGS (Asia) Limited and Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
Publicly available information on the soils of the southern China continental shelf has been sparse. A map of the soils is the fundamental tool in understanding the palaeoenvironment of the shelf.
In recent years at least 20 fibre optic telecommunications cables have been laid across the southern China continental shelf. At least 15 more have been proposed and are at some stage of the planning/design process. Among them, these cables span much of the continental shelf, as shown in Figure 1. Before installation of a cable, seabed soils are mapped along each of the routes; many of them have a detailed survey over a corridor a kilometre or more wide around the cable route. The main survey techniques have been seismic profiles, side scan sonar and swath bathymetry, supported by some combination of cone penetrometer tests (CPTs), gravity cores and grab samples. The main purpose of the surveys has been to map possible obstructions (such as the wreck shown in Figure 2 below) and the soil characteristics. The cable engineers require detailed information on the soils so they can bury the cable beneath the seabed, to protect it from damage, for example from fishing activity.
Each survey of a cable route corridor provides an intriguing section across the shelf, from the landfall to the continental slope and beyond. Not all of the cable owners have provided access to the survey reports for this paper, but there is sufficient information so that together a picture can be built that clearly shows the distribution of soils that provide a record of the palaeogeography. The main features of the poster map are:
- The very soft to soft Holocene CLAYS that fan out from the Zhu Jiang Kou (Pearl River Estuary) in the western half of the area covered by the map. In some areas, the Holocene CLAYS contain biogenic gas and there is at least one area where there is a significant seep of natural gas, apparently from a deeper source. The CLAYS overlie Tertiary alluvial soils, commonly showing complex working/reworking by fluvial/estuarine processes. This suggests that the palaeo Zhu Jiang meandered widely across the shelf when the sea level was lower.
- SAND and GRAVEL with sand waves in the eastern part of the shelf.
- A regional line of coarse sediments and exposed rock that runs along the top of the continental slope at around 100 m to 150 m depth. Samples of this tough material are difficult to obtain, but the small samples will typically contain well-rounded GRAVEL. This feature has been interpreted as a palaeo-beach from periods of lower sea levels.
- Rough, irregular, terrain down the continental slope, with rock outcrops, slumps and sediment flow gullies.
Stratigraphy and sea-level history of the late Pleistocene
Till J.J. Hanebuth 1, Karl Stattegger 2 and Yoshiki Saito1 - 1 AIST, Geological Survey of Japan, MRE, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8567, Japan - 2 University of Kiel, Institute for Geosciences, Geology, Olshausenstr. 40-60, 24113 Kiel, Germany
The high-resolution reconstruction of an ancient fluvial/shelf system and of climatically induced sea-level fluctuations during the late Pleistocene is favoured by two main conditions: 1. A wide shelf extension with the corresponding low gradient and a high fluvial sediment input which leads to an exploded resolution of wide lateral facies shifts; 2. An extended sediment record in combination with seismic surveying of local structures reflecting the high complexity of the former coastline-related environment and its shifts. Tropical siliciclastic shelves additionally offer special conditions, such as tidal mangroves and a high precipitation ratio (monsoon). The tropical, siliciclastic Sunda Shelf (Southeast Asia) is one of the largest shelf ramps in the world. Due to its semi-enclosure during sea level lowering, only a few but large river systems drained the exposed plain into eastern direction. We present the first reconstruction for the central Sunda Shelf on the basis of numerous sediment cores and shallow-seismic observations.
Stratigraphic architecture over the past three 100 ka sea-level cycles
The nine distinguishable successive, seismic/sedimentary units (within the uppermost 80 m below seafloor) are mainly associated with marine (delta) progradation across the shelf. Three major seismic surfaces represent sequence boundaries (associated with marine isotope stages [MIS] 8, 6 and 2) and go together with soil formation and channel incision. Higher-order erosional discontinuities, e.g. transgressive surfaces, are common, accompanied by shifts of the seismic/sedimentary facies/units. The seismic record, correlated to a generalized sea-level curve, covers the time interval of the past 280 ka (Hanebuth et al., submitted).
The time-resolution (and the probability of preservation) increases with younger deposits and discontinuities and, therefore, the post-Eemian (after 116 ka) deposits show a higher diversified history. Regressive deposits dominate the stratigraphic architecture either as continuous and thick strata balancing the former physiography or as detached depression fills. These detached bodies in -80 to -100 m water depth may have been related to minor sea-level fluctuations, when sea level reached a critical range close to the average depth of the central shelf, particularly since MIS 4 (Hanebuth et al., 2001). Transgressive deposits occur only as isolated fills of incised channels and as thin marine blanket. A succession of facies from terrestrial, estuarine and nearshore environments deposited during the past deglacial transgression and reflects rapid lateral and stratigraphic shifts (Hanebuth and Stattegger, 2001).
Late Pleistocene sea-level history
Radiocarbon dates of coarse-grained plant remnants and in situ mangrove roots from deposits that have formed directly within the paleo-tidal zone (less than 2 m) were used for a sea-level reconstruction. The recorded sea level was not deeper than -120 m during the Last Glacial Maximum lowstand (Hanebuth et al., 2000). This implies that the glacial coastline did not reach the shelf break (at -180/-220 m). A calculated isostatic effect (Lambeck et al., 2001) amounts 4 m (to 8 m distally) resulting in a lowstand not exceeding -125 m. The subsequent deglacial rise took place at gradually increasing acceleration, comparable to other sea-level curves, until the abruptly starting interval of very rapid rise associated with Meltwater Pulse 1a: the rates averaged out 5.3 m/100 yr during the phase from 14.6 to 14.3 cal. ka and the pulse was, therefore, shorter and faster than previously assumed. Prior to the Last Glacial Maximum, sea level obviously decreased gradually since about 40 ka and reached -110 m already at about 30 ka, earlier than observed in other sea-level records (Hanebuth et al., 2001).
Hanebuth, T., Stattegger, K. and Grootes, P.M. (2000). Rapid flooding of the Sunda Shelf - a late glacial sea-level record. Science 288: 1033-1035.
Hanebuth, T.J.J. and Stattegger, K. (2001). The stratigraphic evolution of the Sunda Shelf during the past fifty thousand years. SEPM Spec. Publication (in press).
Hanebuth, T.J.J., Schimanski, A. and Stattegger, K. (2001). Late Pleistocene forced regression deposits on the Sunda Shelf (SE Asia). International J. Earth Sci. (in press).
Hanebuth, T.J.J., Stattegger, K. and Saito, Y. (submitted) The architecture of the central Sunda Shelf (SE Asia) recorded by shallow-seismic surveying. Geo-Marine Letters.
Lambeck, K., Yokoyama, Y. and Purcell, T. (2001). Into and out of the Last Glacial Maximum: sea-level change during oxygen isotope stages 3 and 2. Quaternary Science Reviews (in press).
Paleogeography and early human adaption of the Queen Charlotte
Islands, Canada:drowned landscapes, paleo-coastlines, and paleo-marine
habitats R. Hetherington1,2,.J.V. Barrie, R. Reid, R. MacLeod
and R. Kung - 1 Department of Geography, Biology, University of Victoria,
Petch Building 116, 3800 Finnerty Road, Victoria, BC, Canada, V8P 5C2
- 2 Geological Survey of Canada, 9860 West Saanich Road, P.O. Box 6000,
Sidney, BC, Canada, V8L 4B2
Paleogeographic reconstruction of the northeast Pacific continental shelf, specifically the Queen Charlotte Island (QCI) region on the west coast of Canada, elucidates changes to coastline extent, littoral dynamics, paleohabitats, and identifies a land-bridge between QCI and the mainland created by the narrowing and closure of Hecate Strait subsequent to the Last Glacial Maximum (LGM). A global warming event ensued LGM that saw the melting of extensive continental ice sheets, precipitated eustatic and isostatic crustal adjustments resulting in sea levels rising over 200m along the British Columbia (BC) mainland, and dropping more than 150m in the adjacent QCI archipelago, located only 100 km offshore. The reason for these significant and contrasting alterations in sea level, was the elaborate interplay between ice thickness, ice extent, and flexural rigidity of the lithosphere, which combined to produce a complex and rapidly changing pattern of glacio-isostatic – and to a lesser extent hydro-isostatic – uplift and subsidence. Rapid ablation of ice sheets concomitant with large fluctuations in sea level heralded profound changes in the variety and extent of coastal landforms, and a rapidly changing web of marine and non-marine animal and plant species in the QCI region.
Evolving coastal landscapes are charted on a series of paleogeographic and isostatic land-surface change maps that contain the analysis and integration of a variety of databases. Databases include habitat characteristics and age of molluscs collected from submarine grab samples, submarine cores and raised beaches, sedimentological analysis of cores and raised beaches, seismic reflection records, sidescan sonar records, and bathymetry. Data integration, spatial analysis, and modeling were accomplished using a combination of geographic information systems (GIS) belonging to the ArcInfo software family. The maps illustrate temporal changes in the magnitudes and extent of crustal flexure as a forebulge migrated across the region. At ca. 14,000 yr BP ice sheet retreat in, and west of Hecate Strait, induced uplift east of northern QCI. During the period 13,250 to 12,250 yr BP, a forebulge created uplift throughout Queen Charlotte Sound and Hecate Strait, synchronous with downwarping at Port Simpson on the BC mainland. Maximum sea level lowstand in the region occurred between 11,750 and 11,250 yr BP, exposing a coastal plain which extended north, south, and eastward from the QCI, connecting the islands to the BC mainland. This connection was maintained until waning ice sheets reduced the forebulge between 10,250 and 9,250 yr BP. The consequent rise in eustatic sea levels resulted in inundation, impeded drainage of lowland areas, lacustrine formation, and flooding of depressed mainland regions. Subsequent to forebulge collapse, relative sea levels rose quickly resulting in sea levels consistent with today by approximately 9,000 yr BP.
The presence of a land-bridge, the closure of Hecate Strait, the flooding and rapid sea-level rise in Hecate Strait, and the appearance and disappearance of a flat coastal plain had a significant impact on the ability of human, faunal and floral populations to migrate and colonize coastal areas. Paleoenvironmental reconstruction, yields insights into water temperature, de-glaciation patterns, timing and distribution of productive intertidal habitats, and the ability of species to recolonize rapidly changing environments. This research tests the theories that a glacial refugia existed in this region and that productive edible resources were available to support an early people who migrated across the Bering land-bridge and down the west coast of British Columbia en-route to colonizing North and South America.
Glaciers, lowered sea-levels and early humans on Canada’s
Continental shelves Heiner Josenhans - Geological Survey of Canada
(Atlantic), Box 1006, Dartmouth, N.S., Canada
The most dominant processes shaping the continental shelves of Canada in the last 20,000 years have been glaciation and the subsequent glacio-isostatically driven sea-level changes. The spatial pattern and timing of ice margin retreat is shown, based on a combination of multibeam bathymetry, seismic reflection profiling and dated piston cores obtained from ice marginal deposits. Digital terrain images of ice marginal deposits are illustrated and show the detailed configuration of these complex paleo-ice margins. Isostatic crustal rebound which followed ice retreat is documented from site specific study areas from both coasts of Canada. These detailed studies are based on interpretations of multibeam, seismic reflection and cores and also include archeological investigations of the sampled material. The results show that dramatic changes in sea-level resulting in subaerial exposure of vast areas of the continental shelves. On the west coast in Northern British Columbia, we have documented sea-level lowering to 150 m below present about 10,300 years ago. Human stone tolls have been recovered at a depth of 52 m below present. On the east coast in the Gulf of St. Lawrence we have documented sea-level lowering to a depth of 70 m, 9,500 years ago. Stone tools (3000 artifacts) recovered from coastal areas in the Gulf of St.Lawrence suggest a human presence on these drowned landscapes. Sea-level recovery to the present coastline has resulted in the transgression of large areas of the Canadian continental margins. The wave reworking of glaciogenic sediments has produced large sand deposits in some areas and a lag gravel armour in others. The fine sediment fraction washed out by the transgression are deposited within the shelf basins. The outcrop pattern of these deposits is illustrated by regional surficial outcrop maps and in more detail at site-specific study areas where multibeam data and bottom samples provide resolution similar to aerial photographs and land use maps. The combination of regional and site-specific studies provide a model of the post glacial evolution of the continental shelves which can be used as a predictive tool for bottom attributes and seabed habitat. These observations provide the groundwork for offshore development as well as fisheries habitat studies and early human history.
Can the distribution of foraminifers in Holocene inner
shelf sediment from the South China Sea be used as typhoon indicators?
Guangqing Huang1 and Wyss W.-S. Yim2 - 1 Guangzhou Institute of Geography, Yellow Flower Hill, Guangzhou 510070, China - 2 Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
The distribution of foraminifers in Holocene inner shelf sediments from the South China Sea was investigated to determine their potential as typhoon indicators. Six boreholes ranging in seabed depths from 3.8 to 26.2 m were studied including three from the inner Pearl River Estuary, two from the middle Pearl River Estuary and one from the open shelf. Two types of storm beds were found: 1. Siliciclastic-dominated beds mainly of fluvial and/or beach sands occurring in shallow water. 2. Bioclastic-dominated beds formed by the resuspension of seafloor sediment occurring in deeper water. The later is found to possess a higher diversity of foraminifers compared to non-storm beds due to mixing of estuarine sediments during typhoons. The maximum number of storm beds found in the cores from the inner and middle Pearl River Estuary studied is seventeen representing an average of just over two typhoons per thousand years. No variations in foraminiferal distribution have been found in the borehole from the open shelf to indicate typhoon signatures at this location. This was attributed to the lower sedimentation rate and the greater reworking by physical and biological agents. The open shelf without the supply of fluvial sediments is concluded to be unsuitable for providing typhoon signatures.
A high-resolution record of the late glacial maximum event
in the western Black Sea
Gilles Lericolais1, Nicolas Panin2, François Guichard3 and Candace Major4 - 1 IFREMER, Centre de BREST, BP 70, F29200 Plouzané cedex, France - 2 GEOECOMAR, 23-25 Dimitrie Onciul Str , BP 34-51, Bucuresti, Romania - 3 LSCE, CNRS-CEA, Avenue de la Terrasse, BP 1, 91198- Gif-sur-Yvette cedex, France - 4 Lamont-Doherty Earth Observatory, Palisades, NY 10964, U.S.A.
The spring 1998 "Blason" expedition of the R/V LE SUROIT collected 38 piston cores on the Romanian continental margin of the western Black Sea. A transect between the shelf (-50 m) and the upper slope (-380 m) was examined to find evidence of a late glacial lowstand and to obtain a record of continuous sedimentation from the peak glacial period to modern times. These cores were collected along a seismic reflection profile traversing the continental margin from the inner shelf to the deep abyssal plain.
The goal of this study is to explore the relationship between sea-level changes, climate, and sedimentation by studying closely-spaced samples in sequences with high sedimentation rates aided by radiocarbon dating. Proxies for climate are the stable isotopes of oxygen and carbon and the composition of clays and heavy minerals from fluvial sources. Previous studies have been carried out on cores from the basin floor (Deuser, 1972; Ross and Degens, 1974) in which three lithologic units were defined: 1. The youngest, Unit 1 (0-3 ka), is a laminated, coccolith-bearing sapropel. 2. Unit 2 (3-7.2 ka) is a varved sapropel with a very minor carbonate content, with a characteristic thin basal aragonite layer. Organic carbon contents are high in Unit 2, representing an anoxic environment which (Jones and Gagnon, 1994) showed to have developed simultaneously in all depths from -2,200 to -200 m. 3. Unit 3 is a laminated lacustrine clay with characteristically light d18O and notably higher carbonate content than Units 1 and 2. The basin floor cores bottomed in Unit 3 and the base of the core is dated ca. 25 ka BP. The dinoflagellates of Units I and 2 are euryhaline marine species and those of Unit 3 are very low salinity, stenohaline species that are contemporaneous with the Neoeuxine freshwater mollusks reported by Russian authors from the shelf (Nevesskaja, 1965; Wall and Dale, 1974). The calcareous microfossils in Unit 3 are entirely reworked species from the Eocene and Cretaceous, but those in Units 2 and 3 are autochthonous marine species (Bukry, 1974). The transition from lake to marine conditions was a marked event and was attributed to the entry of water from the Mediterranean caused by the eustatically rising sea level to the point where it could spill in through the Bosporus Strait (Ross and Degens, 1974).
The sediments on the western Black Sea continental slope (Site BLKS9810) provide a high-resolution record the latest Pleistocene events in the Black Sea. The dominant variability in the chosen stratotype core represents climate and Black Sea level instability at the last glacial termination. The events seen in the measured parameters (% carbonate, d18O and d13C, and clay mineralogy), together with 14C dates from mollusc shells, suggest that the majority of the instability occurred during the Younger Dryas to Cochran Cold Event.
Bukry, D. (1974). Coccoliths as Paleosalinity Indicators; evidence from Black Sea. In: Degens, E.T. and Ross, D.A. (eds.). The Black Sea; Geology, Chemistry, and Biology. American Association of Petroleum Geologists, Tulsa, pp. 353-363.
Deuser, W. G. (1972). Late-Pleistocene and Holocene history of the Black Sea as indicated by stable isotope studies. Jour. Geophysical Research 77: 1071-1077.
Jones, G. A. and Gagnon, A. R. (1994). Radiocarbon chronology of Black Sea sediments. Deep Sea Research 41(3): 531-557.
Nevesskaja, L. A. (1965). Late Quaternary bivalve mollusks of the Black Sea: their systematics and ecology. Akad. Nauk SSSR Paleont. Inst. Trydy 105: 1-390.
Ross, D. A. and Degens, E.T. (1974). Recent sediments of the Black Sea. In: Degens, E.T., Ross, D.A. (eds.). The Black Sea - Geology, Chemistry and Biology. Amer. Assoc. Petrol. Geol. Mem., Tulsa, pp. 183-199.
Wall, D. and Dale, B. (1974). Dinoflagellates in the late Quaternary deep-water sediments of the Black Sea. In: Degens, E.T., Ross, D.A. (eds.). The Black Sea - Geology, Chemistry and Biology. Amer. Assoc. Petrol. Geol. Mem., Tulsa, pp. 364-380.
Palaeodeltas during the last glacial period in the outer
shelf of the East China Sea
Shuanglin Li and Shaoquan Li - Qingdao Institute of Marine Geology, Qingdao 266071, China
Core EA1 is located in the Xihu Depression of the outer shelf of the East China Sea. The delta was developed at the range of 0.36-29.65 m in the core, which includes prodelta composed of clay and silt from 29.65 to19.88 m, sandbar from 19.88 to 11.33 m, interdistributory bay consisted of clay with little silt from 11.33 to 8.43m, and sandbar from 8.43 to 0.36m.The lateral shifting of river mouth resulted in alternation of sandbar and interdistributory bay. Because of rapid sea-level rise during the Holocene, the speed of transgression exceeded the rate of progradation of the delta, and no delta plain facies was developed on the sandbar. Sediment in the Holocene only reached a thickness of 0.36 m. Radiocarbon dating of the littorial-neritic sediment beneath the delta gave an age of 21,690 ± 780 yr BP indicating that the delta was formed during the last glacial period.
Core EA5 is located in the Oujiang Depression of the outer shelf in East China Sea. The paleodelta exists at the range from 3.4 to 53.43 m in the core, which include sandbar from 53.43 to 42.23 m, sandbar-interdistributory bay sediment composed of sand with a little mud from 42.23 to 29.39 m, interdistributory bay from 29.39 to 8.63 m, tidal flat of mud and with a little sand from 8.63 to 3.40 m. There is a clear erosion surface at the depth 3.4 m in core EA5. 14C ages: 18,920 ± 720 to 22,000 ± 710 yr BP indicate that the delta was formed in the last glacial period.
There is a series of depression basins developed in the shelf of east China Sea, which has obvious inherit in geological evolution. Some basins were not come out sea surface in the last glacial period, even in the last glacial maximum, and become main deposition areas of the terrigenous materials carried by some rivers such as Changjiang River and Yellow River at the last glacial period.
Geochemical tracing of sediment sources of the deltas suggest that the sediments are similar to the modern sediments of Changjiang River and Yellow River, and more close to that of Yellow River, which can be given two explanations, one is that Yellow River come into the basins in the last glacial period, other is that Changjiang river in the last glacial period is similar to the modern Yellow river, differ from the modern Changjiang River.
The presence of paleodeltas in the outer shelf of east China Sea at the last glacial period suggest that sea water did not move at the contours of 130 m everywhere, and there were some water existed in some depression basin which become main deposition areas of sediment carried by Changjiang River or Yellow River.
Quaternary transgressive and regressive depositional sequences
of the East China Sea
Zhenxia Liu, Ping Yin,1 Serge Berne,2 Alain Trentesaux3 and Kelin Zhuang4 - 1 First Institute of Oceanography, SOA, Qingdao, 266061,China - 2 IFREMER, Centre de Brest, DRO/GM, BP70, 29280 Plouzane, France - 3 Sedimentologie et Geodynamique,UMR8577 CNRS, Université de Lille, France - 4 Institute of Marine Geology, Qingdao 266071, China
Quaternary sea level of the East China Sea (ECS) fluctuated with global climate changes. During sea-level rise, Pacific tidal waves impacted actively on the ECS continental shelf and strengthened the reciprocating tidal currents with main axis of NW-SE direction. Tidal sand ridges were modeled under the strengthening currents from the outer shelf to the inner shelf and covered vast areas of the ECS continental shelf. They represent large transgressive deposits visible in seismic profiles. In response to sea level fall, Yangtze River delta prograded seaward and built massive subaqueous deltas in the distance of hundreds of kilometers across the shelf, forming regressive sequences on the seismic profiles.
The prograding subaqueous deltas overlaid on the sand ridges, then were overlaid by fluvial, lacustrine, and delta plain deposits behind the shoreline during sea-level fall or lowstand. These upwards developing sand ridges, delta and terrestrial deposits form a well-preserved trangressive-regressive system corresponding to the sea-level changes. The preservation of the sequences corresponding to each glacio-eustatic cycle was favoured by a relatively large subsidence.
Several transgressive and regressive depositional sequences are vertically distributed on the ECS continental shelf, especially the outer shelf, corresponding to high frequency sea-level changes during the Quaternary, and the stratigraphic sequences are consistent with the SPECMAP curve (Martinson et al., 1987). Several stages of tidal sand ridges were modeled during Quaternary sea-level rises, among them, sand ridges of Oxygen Isotopic Stage (OIS) 8-7 (U10), OIS 6-5 (U6.1) and OIS 2-1 (U2). They can be easily recognized on the high-resolution seismic profiles. In addition, large river deltas developed following each sea-level fall. Within Mid-Late Quaternary, four stages of large deltas can be recognized on the high-resolution seismic profiles, among then, U5 and U6 were deposited in the middle and early of Late Pleistocene by Yangtze River (Liu et al., 2000), U9 and U8 could be deposited during Middle Pleistocene according to the sea-level changes model.
Liu, Z.X., Berne, S., Saito, Y. et al. (2000). Quaternary seismic stratigraphy and paleoenvironments on the continental shelf of the East China Sea. J. Asian Earth Sciences, 18: 441-452.
Martinson, D. G. et al. (1987). Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000 year chronostratigraphy. Quaternary Research 27: 1-29.
Sequence stratigraphy of the Roussillon Shelf, southwest
Gulf of Lions, France
F.J. Lobo,1 M. Tesson, B. Gensous2 and F.J. Hernández-Molina3 - 1 CIACOMAR-Univ. do Algarve, Avda. das Forças Armadas s/n, 8700-311 Olhão, Portugal - 2 CEFREM-ERS1745-Univ. de Perpignan, 66860 Perpignan, France - 3 Faculty de Ciencias del Mar-Univ. de Cádiz, 11510 Puerto Real (Cádiz), Spain
Shelf stratigraphic architecture is well documented for the Rhone (Tesson et al., 1990; Gensous and Tesson, 1996) and the Languedoc (Tesson et al., 2000) sectors of the Gulf of Lions. Quaternary deposits in these shelves are represented by a middle-outer shelf sedimentary wedge, which is constituted by two types of deposits (Tesson, 1996; Tesson et al., 2000): 1. Regional prograding units (RPU) are laterally extensive wedges, characterised with low-angle prograding configurations; 2. Intercalated units (IU), which are located between RPU and are constituted of several, patchy deposits over the shelf with currently high-angle prograding clinoforms. The Roussillon shelf (SW part of the Gulf of Lions) is less studied, and shows several stratigraphic features that differ from other sectors. This paper provides the first description of its sedimentary structure, taking special consideration of controlling factors and peculiar sequence stratigraphy characteristics. This work has been based on the analysis of a dense grid of high-resolution seismic profiles collected on the Roussillon shelf during seven oceanographic surveys (between 1994 and 1997). The seismic source was a 50 Joules Minisparker.
Twelve seismic units have been identified in the Roussillon shelf. Those units have been classified according to their distribution and internal structure in several types, considering the already proposed terminology for the other Gulf of Lions sectors:
1. Regional prograding units (RPU), characterised by a widespread shelf distribution. They are subdivided in: i) Shelfal RPU: they are distributed over the middle-outer shelf. Main depocenters are on the middle shelf, showing an elongated, coastline parallel pattern. Dominant seismic facies are low angle (<18) prograding, and intercalated wavy facies are also frequent. They are interpreted as distal portions of coastal bodies, deposited in a moderate to low energy marine environment. ii) Shelf-break RPU: they are distributed over the outer shelf-upper slope, with thickness increasing steadily seawards. Low angle facies evolve seaward to high-angle facies (>28). They are locally affected by erosive channels. They are interpreted as distal facies of coastal deposits prograding over the upper slope.
2. Intercalated units (IU), mainly characterised by discontinuous, patchy distribution over the shelf, and dominated by high-angle progradational configurations. They are subdivided in: i) Discontinuous IU, constituted by several unconnected deposits, which are generally disposed over the outer shelf/shelf-break, middle shelf and inner shelf. These units are attributed to a large variety of littoral deposits. ii) Middle shelf continuous IU: they have lateral continuity and progradational configurations, normally high-angle, but they may evolve seaward to low-angle configurations. Occasionally, erosive channels are determined at their upper boundary. These deposits are attributed to high-energy environments, such as shorefaces that may evolve seaward to shelf muds.
2. Regional aggrading unit (RAU); it is the recentmost unit, characterised by sub-horizontal internal configuration and wedge external shape, distributed over the inner-middle shelf. It is interpreted as fluvially-derived sediments.
3. These seismic units comprise four depositional sequences (B, C, D, and E) mainly constituted by regressive Lowstand Wedges and secondarily by Transgressive Deposits. Two main types of cycles can be proposed to explain their development: a fourth order (about 100,000 years) and a fifth order cyclicity (about 20,000 years). However, the different preservation of RPU and existence of continuous IU indicates that other factors have also controlled the development of this shelf sector. We propose that reactivation of pre-existing structures could have influenced shelf subsidence and therefore permitted the good preservation of mid-shelf deposits through much of the shelf history. Besides, the influence of submarine canyons on shelf deposition should be taken into account in this case.
Several aspects in terms of sequence stratigraphy interpretation can be put into question. Between them, we would like to discuss the following:
1. No distinction of a regressive/lowstand boundary. Regressive deposits on the Roussillon shelf do not show significant internal boundaries, and they are only differentiated by the degree of preservation. Thus, shelfal RPU are best preserved in middle shelf settings, whereas shelf-break RPU are better preserved in marginal settings.
2. Sequence interpretation of IU is still open. Stratigraphic characteristics of discontinuous IU drive us to interpret them as transgressive deposits, but it is not clear if outer shelf deposits would be related to maximum sea-level lowstands or to initial stages of sea-level rises. This interpretation would be based on their attribution to a particular depositional system, which is questioned. Besides, the occurrence of continuous IU laterally related to low-angle deposits is a particular stratigraphic feature of this shelf, as it would provide evidence of forced regressions or of superimposed sea-level variations.
The most distinct, significant surfaces which are identified in seismic sections establish the boundaries between RPU and IU, and they are considered as transgressive surfaces. By contrast, sequence boundaries are identified by downlaps of RPU over ancient deposits. These considerations have genetic implications, in the sense that these surfaces are related to sea-level changes of similar magnitudes but very different duration.
Gensous, B. and Tesson, M. (1996). Sequence stratigraphy, seismic profiles, and cores of Pleistocene deposits on the Rhône continental shelf. Sedimentary Geology 105: 183-190.
Tesson, M. (1996). Contribution à la connaissance de l’organisation stratigraphique des dépôts d’une marge siliciclastique. Étude de la plate-forme continentale du Golfe du Lion. Mémoire d’Habilitation à Diriger des Recherches: 102 pp.
Tesson, M., Gensous, B., Allen, G.P. and Ravenne, C. (1990). Late Quaternary Deltaic Lowstand Wedges on the Rhône Continental Shelf, France. Marine Geology 91: 325-332.
Tesson, M., Posamentier, H.W. and Gensous, B. (2000). Stratigraphic organisation of Late Pleistocene deposits of the western part of the Rhone shelf (Languedoc shelf) from high resolution seismic and core data. Amer. Assoc. Petrol. Geol. Bull. 84/1: 119-150.
Post-LGM sedimentation on the outer shelf/upper slope of
the northernmost part of the Sao Paulo Bight, southeastern Brazil
Michel Michaelovitch de Mahiques, Ilson Carlos Almeida da Silveira,1 Silvia Helena de Mello e Sousa2 and Marcelo Rodrigues1 - 1 Department of Physical Oceanography, Institute of Oceanography, University of São Paulo - 2 Department of Geology, Federal University of Paraná
In this paper we present a first interpretation of post-Last Glacial Maximum (LGM) sedimentation on the outer shelf and upper slope of the northernmost part of the São Paulo Bight, south-eastern Brazil. Main attention was given to the role played by the dominant water-mass dynamics during the Holocene sedimentation.
The study area comprises the northernmost part of the São Paulo Bight, southeastern Brazil. The São Paulo Bight is the arc-shaped part of the southeastern Brazilian margin extending from 23oS to 28oS. In the study area the Brazil Current develops a convoluted pattern of meanders on Cape Frio surroundings. The reason is the change in the Brazilian coastline orientation, which dynamically favours the formation of a clockwise meander on BC in an attempt to conserve angular momentum. The consequence is a vortical structure with upwelling associated to its southernmost part and downwelling in its northernmost part. A counterclockwise meander is often found downstream of the Cape Frio meander and a Topographic Rossby wave pattern is observed inside the São Paulo Bight. The wave pattern is not stationary and is thought to be advected southward by the BC, but, the structure depicted in Figure 2B is recurrent.
Fifteen box cores were collected during the 1997/1998 austral summer on board the R.V. “Prof. W. Besnard”. Each core was described and sampled continuously at intervals of 2 cm. Samples were analysed for organic carbon, total nitrogen and sulphur, d13C and d15N in sediments and d18O and d13C in Globigerinoides ruber tests. Also AMS 14C dating was done in two samples of each box-core.
Results show slight but significant variations in calcium carbonate, total sulphur and nitrogen contents as well as carbon and oxygen isotope ratios. Sedimentation rates, varying from 5 to 184 mm.kyr-1 are controlled by the shelf and upper slope morphology, the Brazil Current meander dynamics, and the Coastal Water offshore motion.
The steeper continental slope areas coincide with the dominant onshore flow of the BC meanders, and are characterised by low sedimentation rates. Sedimentation rates are greater near Cape Frio and south-east of São Sebastião Island. In the former case, the interaction between coastline change and wind direction favours the upwelling of the SACW and, consequently, more conspicuous organic matter deposition. In the latter case the dominant offshore flow of the BC meanders is enhanced by the CW motion, and consequent terrigenous sediment transport towards the outer shelf. This depositional process is probably responsible for the development of an arc-shaped deposit located on the upper slope. Further shallow seismic and coring studies are needed to confirm this hypothesis.
During the Last Glacial Maximum the combination of shelf morphology and stronger trade winds may have favoured a widening of the Cape Frio upwelling area as well as modifications in the displacement for the Brazil Current. In adition the progressive Holocene sea-level rise was responsible for a perceptible decrease in the Coastal Water influence on the outer shelf and slope, expressed by a lowering of terrigenous sedimentation. Finally, data show that for the last 5,000 years the shelf of the study area was subjected to an increase in the terrigenous input.
Seismic and sedimentological characters of a 5th-order
depositional sequence formed during last glacio-eustatic cycle
E. Martorelli, F. L. Chiocci1 and G. Ercilla2 - 1 Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza” - 2 Instituto de Cièncias del Mar, CSIC, Barcelona
Shelf deposits formed during last glacial-interglacial sealevel changes are commonly studied both with direct (seafloor sampling) and remote (high-resolution seismics and side scan sonar) methods. However, due to the very fast sea level rise rate (from lowstand minimum at ~120m to its present position in some 10,000 years), usually transgressive deposits are very thin or absent and the study is mainly focused on highstand deposits. Thus, only areas with very high sedimentation rate and favourable geomorphology hindering erosion during transgression, are favourable to study the shelf paleo-environmental evolution during last de-glaciation emicycle.
For these reasons the Calabro-Tyrrhenian shelf is extremely interesting. Its location on a very young and tectonically active passive margin, caused the sedimentation of a thick postglacial blanket (up to 60 m), due to the fast erosion of the Coastal Range that was uplifting during the whole Quaternary. Depositional sequences are very thick especially were there are not subaerial basin trapping the Holocene sediments.
In Calabria the postglacial depositional sequence is made up of both transgressive and highstand deposits in similar proportions. The high thickness of transgressive deposits is very unusual; elsewhere in fact transgressive deposits are extremely thin or absent, even in areas well-fed as in front of the main rivers (as Tevere, Arno, or Volturno Rivers).
On the continental shelf between C. Bonifati and the town of Briatico an integrated seismic/coring data study was undertaken, to reconstruct the environmental evolution during last 20,000 years. To accomplish this goal, 10 cores were collected along 4 transects in selected sites located on the basis of a detailed seismostratigraphic interpretation (some 1,000 km on an area of some 400 km2).
Data were interpreted according to sequence stratigraphic principles (Posamentier and Vail, 1988), modified for the specific characters of the Late Quaternary glacio-eustatic cyclicity (Chiocci et al., 1997).
The object of the analysis is a still-forming 5th order depositional sequence resting on type-one regional sequence boundary formed during last glacial lowstand (Würm unconformity). Within the sequence a transgressive systems tracts (made up of up to three parasequences in retrogradational setting) and a highstand systems tracts (made up of up to two aggradational/progradational parasequences) were defined.
Gravity core analysis (grain size, carbonate, magnetic susceptibility, microscope analysis of the sandy fraction), shows that the transgressive systems tracts is made up of coarse sediment (sorted sand or sandy gravel passing upwards to sand) mainly siliciclastic (quartz and feldspars are the main components). As far as the highstand systems tracts is concerned, in most of the cores it is made up of a fining-up interval with silty-sand passing to sandy- or clayey-silt, passing thus to finer and more homogeneous sediments; sand fraction of the highstand deposits is represent mainly by mica and light minerals.
In the cores more close to the coast, the fining-up interval is overlaid by a coarsening-up interval, likely to represent the toe of the present-day littoral prism.
It is possible that the highstand fining-up interval was created by the reworking of the underlying transgressive deposits with the formation of palimpsest sediments (Swift et al., 1971), slowly loosing the coarse fraction with time.
Within the highstand systems tract the variations are always gradual. Only in one core, located on the maximum thickness area (offshore a span of coast were several streams are present), sandy and silty laminae are bounded by erosional surfaces, likely to have been produced by the activity of the small but powerful streams debouching on the coast.
Palaeoindian archaeological evidence and two cases of land
bridges in southern South America
Hugo G. Nami - CONICET-INGEODAV, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas, Físicas y Naturales (Universidad de Buenos Aires), Ciudad Universitaria (Pabellón II), 1428 Buenos Aires, República Argentina
Archaeological evidence shows that the first human settlers in the New World had adaptive and technological diversity at ca. 11.5-9.5 ka. At that time, many South American paleoindians in their weaponry used a very distinctive point called, Fell's cave or just Fell. They occur from Ecuador and, probably from Central America, to Tierra del Fuego. A broad diversity of fauna, including mastodon, horse, paleolama and other extinct and extant species were exploited by these people.
During the world's prehistorical colonization process, human migration and dispersal had to surpass many geomorphological obstacles. Very important are those large masses of water, such as rivers, lakes and seas. Then, in certain periods of Earth history, the land-bridges become a most important factors in the global human colonization. Particularly, in the southern cone of South America, the archaeological evidence suggests that people were terrestrial hunter-gatherers that crossed water courses that today, for their extensions are true biogegraphical barriers. Then, here two cases of land-bridges that existed in the outlet area of the Río de la Plata and the Magellan Strait is showed.
Outlet of Río de la Plata
The eastern part of the Southern Cone in the Republics of Uruguay and Argentina at the Buenos Aires province, yielded many Fell points. Beyond typological and technical similarities, recent investigations suggest that some points found in Buenos Aires were probably made with stones coming from sources located in Uruguay (Flegenheimer et al., 2000).
At the present, both regions are separated by the Río de la Plata, a great mass of water that covers almost 35,000 km2 which behaves as a true barrier. However, according to Cavalloto and colleagues (1999, 2001) during the terminal Pleistocene, its outer area and its outlet into the Atlantic Ocean was different from today. Then, responding to glacial-eustatic fluctuations, the geomorphology suffered significant changes related to the transition from ancient fluvial to modern estuarine and littoral environments open to the sea. This fact produced significant variations in the submerged and emerged lands' distribution. During the LGM at the beginning of the last transgressive event occurred at 18 ka, the geomorphologic configuration showed a conspicuous emerged feature such as headland separating two fluvial environments. The one located north of the headland included the ancient Río de la Plata, significatively narrower than today and the other one, the Salado river in the south (Cavalloto et al., 1999 and 2001; Violante and Parker, 1999). When the last transgression occurred, those environments were affected in different ways. While the headland of Punta Piedras-Alto Marítimo watershed stayed like a sub-aerial feature although significatively reduced in size, sea water entered the primitive Río de la Plata installing at its mouth and lower areas a long and narrow estuarine environment, which progressively widened and migrated landwards as the transgression progressed (Cavallotto et al., 1999). At 10 to 8 ka, when the sea level reached ca.15m below present level, the headland, elongated in a west-east direction and its southern margin, open to the sea, developed extensive beaches associated with coastal barrier systems. Presently, its relicts are represented by the submerge La Plata Bank that extends northeastwards at the east and southeast of the Punta Piedras-Alto Martimo watershed (Parker and Violante, 1993). On the other hand the protected area located behind the coastal barrier developed swamps and lagoonal environments. As result, it is believed that those environments (subaerial at that time) reached the proximity of the present Uruguayan coast near Punta del Este, staying separated from it only by the narrow outlet of that ancient Río de la Plata (Cavallotto et al., 2001).
Magellan Strait. Presently, at the southern tip of Patagonia, the island of Tierra del Fuego is separated from the continent by the Magellan Strait. However, during the late Pleistocene and Early Holocene was a part of the continent. The archaeological record yielded evidence that the oldest human occupations settled the island at ca.11 ka. Fell points and megafauna were founded of both sides of the strait (Nami, 1996).
At ca. 12-10 ka sea level was perhaps 60 m below present (Coronato et al., 1999). Then, a land-bridge existed between the Primera Angostura and Segunda Angostura; there glacial moraines of different ages extended, at least in part, across a meltwater discharge, braided stream, with many shallow channels and gravel islands. Before ca. 12 ka the Magellan glacier lobe had receded from the western channels of the Magellan Strait, allowing drainage to the Pacific of a large pro-glacial lake that had previously been ponded within the basin of the central section of the Strait and the Bahía Inútil. During the deglacitaion, they left evident shoreline features that now exist at various levels between 10 and 55 m above modern sea level because of glacioisostacy; All these rise in altitude southward. During this time much of the area may have been dry and apart from the deeper and enclosed hollows no major meltwater river would have flowed thought the Segunda and Primera Angosturas. Only rivers, lakes and marshes occupied the area between Segunda Angostura and Punta Dungeness, since global sea level lay some 100 m more lower. When the global sea level had been elevated only about -70 m the Magellan Strait coast could not be over -40 m height (McCulloch et al., 1997). The land-bridge was subsequently breached again sometime at ca.12 ka, when a pro-glacial lake and its outlet river formed as the Magellan glacier readvanced into the Magellan Strait.
Final remarks. From an archaeological perspective, according the previous outline, the Paleoindian dispersion, at the Río de la Plata occurred before the flowing of the Punta Piedras-Alto Marítimo watershed (Cavallotto et al., 2001). Far south, at the Magellan Strait human populations could be crossing the strait on foot from the continental mainland onto Isla Grande de Tierra del Fuego via the connecting land-bridge, specially in the winter when the lake was frozen or the empty or drainage rivers decrease its size (Clapperton, 1992; McCulloch et al., 1997).
Cavallotto, J. L., Violante, R. and Parker, G. (1999). Historia evolutiva del Río de la Plata durante el Holoceno. Act. Cong. Geol. Arg. 1: 508-511.
Cavallotto, J. L., Violante, R. and Nami, H. G. (2001). Late Pleistocene-Holocene Paleogeography and Coastal Evolution in Mouth of the Río de la Plata: Implicances for the Human Peopling in the South America. Cur. Res. Pleist. 19 (submitted).
Clapperton, C. (1992). La Ultima Glaciación y deglaciación en el Estrecho de Magallanes: Implicaciones para el poblamiento de Tierra del Fuego. An. Inst. Pat. (Se. Cs. Soc.) 21: 113-128.
Coronato, A., Salemme, M. and Rabassa, J. (1999). Paleoenvironmental conditions during trhe early peopling of southernmost America (Late Holocene-Early Holocene, 14-8 ka B.P.). Quaternary International 53/54: 77-92.
Flegenheimer, N., Bayón, C., Valente, M., Baeza, J. and Femenías, J. (2000). Traslado de rocas a grandes distancias. Workshop La Colonización del de América durante la transición Pleistocene/Holoceno, Abst., pp. 17.
McCulloch, R. D., Clapperton, C.M., Rabassa, J. and Currant, A.P. (1997). The Glacial and Post-glacial Environmental History of Fuego-Patagonia. In: McEvan, C., Borrero, L.A. and Prieto, A. (eds.), Patagonia. Natural History, Prehistory and Ethnography at the Uttermost end of the Earth. British Museum Press, London, pp. 12-31.
Nami, H. G. (1996). New assessments of early human occupations in the southern cone. In: Akazawa, T. and Szathmáry, E.J.E. (eds.), Prehistoric Mongoloid Dispersals. Oxford University Press, Oxford, pp. 254-269.
Parker, G. and Violante, R.A. (1993). Río de la Plata y regiones adyacentes. In: Iriondo, M. (ed.), El Holoceno en la Argentina. CADINQUA, pp. 163-230.
Violante, R.A. and Parker, G. (1999). Historia evolutiva del Río de la Plata durante el Cenozoico superior. Act. Cong. Geol. Arg. 1: 504-507.
Clastic sedimentary facies of lowstand sea-level during
the LGM in the continental shelf and shelf-edge of the East Sea, southeastern
Yong A. Park - Ocean Research Institute, Seoul National University, Seoul 151-742, Korea
One hundred and fifteen bottom sediments and two hundred suspended-sediment samples (surface waters and near-bottom waters) were taken and collected from the continental shelf and shelf edge off the southeastern coast of the East Sea, Korea in order to understand unique depositional processes and sedimentary facies in the continental shelf and especially along the shelf-edge (Figure 1).
The investigation results seem to indicate positively that the fine-grained bottom sediments and suspended particulate matters (SPM) restricted mainly to nearshore - inner shelf are characteristic modern (late Holocene) sediments, and the gravelly sandy and sandy sediments with the lower value (<5 %) of mud content on the outer shelf and shelf-edge might be considered to be relict sediments, respectively. In short, the coarse-grained sediments (ancient sandy beach) along the outer shelf and shelf-edge became stranded on the coastal zone quite removed from the present nearshore area. It is further considered that the ancient timing for the sandy beach to become stranded on the coastal zone would be about several thousand years for lowstand of sea-level after the LGM.
It is also worthy to note the results of Q-mode factor analysis for 115 bottom sediments on the shelf, which are interpreted to indicate the shelf depositional processes in relation to Holocene sea-level history.
nfluence of the Holocene palaeoenvironment on shore
protection measures in Flensburg Fjord, Baltic Sea
Klaus Schwarzer - Institute of Geosciences of Kiel University, Olshausenstrasse 40, D - 24118 Kiel
The Baltic Sea is a very young ocean, which developed to its present form after the last ice age. Due to the deglaciation there is an ongoing process of crustal rebound in the northern part of the Baltic Sea while the southwestern and southern coastal areas from Denmark via Germany to Poland suffer from sinking relative to the present sea level. Here, cliffs, mainly built up of Pleistocene deposits, and lowlands alternate. The average rate of coastal retreat is approximately 30 cm/year and many areas are subject to erosion and are endangered by floodings. Therefore coastal defence and shore protection are of high priority.
To understand and predict the development of a coastal area it is necessary to look into its geological history. The fjords and bays in the southwestern part of the Baltic Sea were created by glacier tongues and subglacial meltwater channels during the final stage of the last ice age. They are bordered by lateral moraines, which are mainly striking parallel to the present coastline. Marine influence in the southwestern part of the Baltic Sea started during the Litorina transgression approximately 7,000 years before present (Voipio, 1981). At that time the coastline was very lobate. Due to erosion, transport and deposition a smoothing process of the coastline began and continues until today. The formation of beach ridges led to the development of lagoons and in front of those lagoons a nearshore morphology with a bar and trough system developed in case of sufficient sediment supply. All structures were supplied by material derived from erosion at the coastline and as well from the shallow areas of the seabottom.
The ongoing process of coastal retreat normally leads to continuous reworking of all nearshore deposits combined with a landward displacement of the whole nearshore geomorphological system which consists of beachridges, beaches and nearshore bars and troughs. Nevertheless, remnants of beachridges can be preserved even in shallow waters and under wave influence (Schrottke, 1999) for thousands of years (Schwarzer et al., 2000).
In many cases the aim of shore protection is to keep the coastline at a fixed position, disturbing the landward oriented displacement process of the nearshore geomorphological system. As a result, the gradient of the nearshore slope increases, the width of the nearshore geomorphology system decreases and the zone of maximum energy dissipation moves close to the shore. This process and subsequent consequences will be described in detail using an example from the outer Flensburg Fjord.
A dyke protects a lowland, “Geltinger Birk”, which is endangered by floodings. In the nearshore area a bar and through system is developed which consists of up to 4 bars. The maximum width of this system is 600 m. It is bordered seaward by a steep slope (maximum gradient of 1 to 6), beginning in a depth of -4 m below sea level and flattening in a depth of -12 m below sea level.
Post-last glacial maximum coastline change as a major forcing
of regional hydrodynamic variations: an example from the eastern Brazilian
Silvia H. M. Sousa,1 Michel M. Mahiques,2, 3 Raquel F. Passos,2 Luiz Fernando D'Agostino2 Thomas R. Fairchild,4 Wania Duleba5 , Alberto G. Figueiredo3, 6 and Jürgen Patzold7
1 Departamento de Geologia – Universidade Federal do Paraná - 2 Instituto Oceanográfico – Universidade de São Paulo - 3 Researcher CNPq - 4 Instituto de Geociências – Universidade de São Paulo - 5 Instituto de Biociências – Universidade de São Paulo - 6 Departamento de Geociências – Universidade Federal Fluminense - 7 Fachbereich Geowissenschaften Universität Bremen, Germany
Calcium carbonate, organic carbon, total nitrogen and sulphur contents, foraminiferal analysis, and stable isotope composition of planktonic foraminifera in three box-cores and one gravity-core allowed the reconnaissance of strong changes in the water mass dynamics after the Last Glacial Maximum, on the southern upper slope off Abrolhos Bank, Eastern Brazilian margin.
The Abrolhos Bank is a morphological feature that stands out on the Eastern Brazilian margin. It is characterized by a shallow shelf, covered by extensive Holocene coral and algae carbonate banks, with ages not older than 6,600 yr BP. These banks overlie Pleistocene calcareous bioconstructions which accumulated over older Mesozoic volcanic rocks.
Origin and evolution of the continental giant ‘Chacopampeno’
Shelf, Argentina: their evolution and morphlogy from the Miocene to the present
Roberto Torra - Facultad de Ingeniería, UNNE. Av. Las Heras 727, 3500, Resistencia, Chaco, Argentina
For more than a century and a half geologists have studied the wide Chaco-pampeano plains in order to understand their genesis and evolution. Until now, the action of loessial sediments was the main and accepted hypothesis for the 'Pampeano' shelf sedimentation. In a detailed study of the scarce littoral siliciclastic facies of the Miocene sediments outcropping near the Paraná river gullies, I interpreted the genesis of these sediments and characterize them in many classical and routine ways . Special care was taken in the geochemical signatures of these sandy quartz-arenite-muddy sequence sediments. So I defined the main molecules and 36 trace elements of the marine Miocene siliciclastic sequence called the Ituzaingó Formation.
Over the marine Miocene sequence, there are about of 1 to 18 m of friable soft silty-muddy-very fine sand sediments slightly weathered (sometimes irregularly calcretized). Its relation with the parental underlying rocks (the marine Miocene siliciclastic sequence) is conformable elsewhere. This structural relation is equal and unequivocal over more than 700,000 km2 in the Argentina plains. The magnitude of the marine Miocene transgression was never assumed in their entire continental extension (see Figure 1).
Once the geochemical sampling (about 50 selected samples) had picked out for main and trace elements they were analyzed using XRF and ICP-AES techniques. The samples cover more than 450,000 km2 of this friable-soft overlaying sediments. The results appear showing a close correspondence between the marine Miocene sequence and the overlaying friable-soft Pampeano sediments. Then, correlation between the marine Miocene sequence and the overlaying sediments (called in a very custom terminology as 'Pampeano Formation') was proved as originating from the same sediments completely opposite to the previous loessial origin theory.
It is necessary remark that this formation was assumed as continental and fluvial in origin with the contribution of the hypothetical loess sedimentation plus insignificant volcanic Quaternary ash materials derived from the andesitic volcanic trend of the Andean orogene.
The geochemical result, as well as other routinary sedimentologic studies, shows that the mother rock of the 'Pampeano Formation' is the marine Miocene littoral siliciclastic sequence. This sequence ends upwards into a typical 'Highstand Transgressive System' (HTS). These rocks are obviously fine grained; i.e tempestites lithofacies, muddy lithofacies, typical offshore sand body deposits lithofacies, etc. The 'Pampeano Formation' derived from these Miocene marine sediments, as mentioned above, but they were moderately weathered during Quaternary times as well as they were deeply calcretized and affected by pedogenesis.
The morphological analysis, carried out by means of remote sensing procedures, revealed that the geoforms are extremely young, not more than 20,000 yr BP. This observation reveals a critical change in the area since the end of the LGM. From this time the friable-soft sediments of the area (the 'Pampeano Formation') were modeled mainly by fluvial and pluvial systems but not by aeolian erosion.
>From the point of view of the tectonic setting, a typical initial synrift model is present with it axes into the Paraná-Paraguay fluvial river valley. Extensional faulting and transcurrent faulting are present elsewere in the studied area. Its observation is well defined by means of satellite imagery and correlation of log records as well as an en echelon pattern.
In spite of the points established, many doubts persists in the area. It is largely probable that during Pliocene and/or Pleistocene, or early Holocene, some marine trangressions occurred (i.e. giant tsunamis or erratic sea level changes where eustasy is not well documented). The presence of basaltic pillow lavas outcropping at the west of the area, previosly assumed as Cretaceous in age (Quaternary for this paper), shows that this possibility is feasible. Moreover, the plains studied have an altitude average of only 40 m over the sea level including the western pillow lavas (see Figure 1).
The exceptional soils used for both agriculture and cattle-rising activities that developed in the Argentina plains were improved during the last 3,000-6,000 yr BP (last dated restricted marine transgressions) to the modern period when the climatic conditions are similar, specially in the last 500-1,000 years.
Some main features of the bottom topography and the latest
Pleistocene-Holocene sediments on the shelf of the Tonkin Gulf
Tran Duc Thanh, Tran Dinh Lan, Dinh Van Huy1 and Yoshiki Saito2 - 1 Haiphong Institute of Oceanology, 246 Danang Street, Haiphong City, Vietnam - 2 MRE, Geological Survey of Japan, AIST, Tsukuba, Japan
The Tonkin Gulf is located from lat 16°10'N to lat 21°30'N and from long 105°40'E to long 110°00'E. It is about 150,000 km2 in area, with a mean water depth of 50 m and a maximum depth of 107 m in a depression near its entrance. Along its coastal zone, bays, deltas, estuaries, and lagoons are found. The total catchment area of the rivers flowing into the gulf is about 300,000 km2, including the 155,000-km2 Red River catchment on its west coast. The Red River delta covers an area of 17,000 km2. The active delta-front zone is about 1000 km2 in area and reaches a depth of 20 m. The prodelta is located between 20 and 30 m depth. The islands in the gulf are concentrated in the northwestern coastal zone, and they number more than 3000. Only Bach Long Vy Island, which is composed of Tertiary age bedrock, is located in the central area of the gulf. Surrounding this island, sandy bottom sediments cover an area of 15,000 km2. Outside of the gulf, the edge of the continental shelf is at a water depth of 200 m. Generally, the gulf bottom is gentle with a gradient smaller than 5, rarely 10 to 30 degrees. Some extensive elongated depressions cross depth contours on the gulf bottom. They are considered to be relict, unfilled river valleys incised during the Last Glacial Maximum (LGM). These incised valleys are clearly preserved from depths of 30 to 35 m to the entrance of the gulf. They formed part of the Paleo Red Riversystem, which had a catchment area of about 450,000 km2, or three times larger than that of the present Red River. The valley is traceable to depths of 100 to 110 m. The valley shape is less distinct at water depths of 40 to 60 m (Thanh et al., 1995). Other shallow landforms are mainly small submerged hills consisting of Tertiary age sedimentary bedrock with relative heights above the sea floor of 10 to 15 m or sand ridges with relative heights of 2 to 10 m. The sand ridges are thought to have formed in the nearshore zone, but they are now situated at about 25 to 30, 40, or even 60 m depth.
2. Latest Pleistocene–Holocene Sediments
The latest Pleistocene–Holocene sediments in the gulf have been studied from data of 116 gravity cores collected at over 25 m water depth by the Vietnam–China cooperative surveys between 1960 and 1965. Pleistocene sediments are found in many of these cores from the central and western parts of the gulf between water depths of 26 and 82 m, especially between 32 and 62 m. The Holocene sediments are 10 to 160 cm thick, and most are between 50 and 70 cm thick. In core No. 7401, in front of the Red River mouth at a water depth of 28 m, the Holocene deposits are 160 cm thick. No C-14 dates for the Pleistocene/Holocene sediments in the Tonkin Gulf have yet been obtained. However, the Holocene and Pleistocene sediments are easily discriminated on the basis of characteristic sediment facies.
The latest Pleistocene sediments consist mainly of coarse and fine silt with a median grain size (Md) of 0.01–0.075 mm, but over 30% of the particles are finer than 0.01 mm. Characteristically, they are spotty yellow, brown, and gray in color, weakly consolidated, have low water content (23% on average), low CaCO3 (1.1%) and organic carbon (0.39%), and are rather high in ferrous iron (Lan et al., 1997). The tests of marine diatoms and benthic foraminifers are rather abundant, and planktonic foraminifer tests, marine mollusks, and even coral debris can be found at a very few sites (Thanh, 1995). These sediments are interpreted mainly as river mouth/estuarine to open-bay sediment facies and weathered soil of the terrestrial environment before the Holocene transgression.
Holocene sediments, which lie on the weathering surface of the latest Pleistocene sediments, are distinguished by soft materials, gray and brown colors, coarse to very fine granulometry, including pebbles, sands, silts, and clay and low concentrations of heavy minerals such as hornblende, zircon, epidote, and dolomite. The mean CaCO3 content is 7.47% and the mean organic carbon content is 0.67%, which are higher than those of the latest Pleistocene sediments (Lan et al., 1997). The diatom and foraminifer assemblages of the Holocene and latest Pleistocene sediments are obviously different. The Holocene sediments can be divided into three facies successions (Thanh, 1995). The first succession consists of only one layer, a shallow marine/nearshore or bay-bottom facies, which is distributed in the entrance and in the southwestern parts of the gulf. The second consists of two layers. The lower layer is a salt marsh facies, while the upper one belongs to a shallow marine/nearshore and bay facies. This succession is distributed in the northern part of the gulf, where salt marsh sediments are exposed on the sea bottom in some places. The third consists of three layers, of which the lowest layer belongs to a tidal floodplain facies, the middle one to a delta-front facies, and the upper one to a prodelta facies. This succession is distributed in the central and northwestern parts of the gulf.
3. Gulf evolution since the LGM
The lowest sea level is traceable to at least 110 m below the present sea level on the basis of the topography of the incised valley. The old shoreline was located outside the entrance of the gulf during the LGM. The following rapid transgression reached a depth contour of 60 m at 11 ka, when a small and narrow gulf formed. Sea level rose from the 60-m to the 40-m depth contour more slowly than before, which caused the old river valleys to be partly filled by sedimentation and by the development of salt marshes in the northern part. The maximum transgression occurred at 5 to 6 ka, except for in the vicinity of the Red River mouth, where the maximum transgression occurred at 7 to 8 ka. During the last 5 to 6 ky, the sedimentary processes and the topography have changed greatly in the northwestern part of the gulf, where the Red River delta has prograded rapidly. At present, the influence of sediments supplied by the Red River is very limited beyond 30 m depth. It is manifested by the suspended matter content, and by the grain size and color of the bottom sediments.
Lan, T.D., Hoi, N.C. and Tuan, N.Q. (1997). Structural feature of bottom sediment layer in the Tonkin Gulf. Marine Resources and Environment. T. IV. Sci. & Tech. Pub. House, Hanoi, pp. 65–72.
Son, N.T., Huy, D.V. and Thanh, T.D. (1996). Bottom topography of the Tonkin Gulf. Marine Resources and Environment. T. III. Sci. & Tech. Pub. House, Hanoi, pp. 16–26.
Thanh, T.D. (1995). Lower limit and stratigraphy of Holocene sediments in the Tonkin Gulf shelf. Journal of Earth Sciences (Hanoi) 17: 22–30.
Thanh, T.D., Huy, D.V., Son, N.V. and Cu, N.H. (1995). Preliminary study of an older river bed on the bottom of the Tonkin Gulf. In: Thuc, P.V. (ed): Contributions to Marine Geology. Sci. & Tech. Pub. House, Hanoi, pp. 107–112.
The offlap break position versus sea level: a discussion
Marcello Tropeano,1 Luis Pomar2 and Luisa Sabato3 - 1 Dept. Scienze Geologiche, Università della Basilicata, campus Macchia Romana, 85100 Potenza, Italy - 2 Dept. Ciences de la Terra, Universitat de les Illes Balears, 07071 Palma de Mallorca, Spain - 3 Dept. Geologia e Geofisica, Università di Bari, via Orabona 4, 70125 Bari, Italy
Sedimentary lithosomes with subhorizontal topsets, basinward prograding foresets and subhorizontal bottomsets are common in the geologic record, and most of them display similar bedding architectures and/or seismic reflection patterns (i.e. Gilbert-type deltas and shelf wedges). Nevertheless, in shallow marine settings these bodies may form in distinct sedimentary environments and they result from different sedimentary processes. The offlap break (topset edge) occurs in relation to the position of baselevel and two main groups of lithosomes can be differentiated with respect to the position of the offlap break within the shelf profile. The baselevel of the first group is the sea level (or lake level); the topsets are mainly composed by continental- or very-shallow-water sedimentary facies and the offlap break practically corresponds to the shoreline. Examples of these lithosomes are high-constructive deltas (river-dominated deltas) and prograding beaches. For the second group, baselevel corresponds to the base of wave/tide traction, and their topsets are mostly composed by shoreface/nearshore deposits. Examples of these lithosomes are high-destructive deltas (wave/tide-dominated deltas) and infralittoral prograding wedges (i.e Hernandez-Molina et al., 2000). The offlap break corresponds to the shelf edge (shoreface edge), which is located at the transition between nearshore and offshore settings, where a terrace prodelta- or transition-slope may develop (Pomar & Tropeano, 2001).
Two main problems derive from these alternative interpretations of shallow-marine seaward prograding lithosomes:
1. both in ancient sedimentary shallow-marine successions (showing seaward prograding foresets) and in high-resolution seismic profiles (showing shelf wedges), the offlap break is commonly considered to correspond to the sea-level (shoreline) and used to infer paleo sea-level positions and to construct sea-level curves. Without a good facies control, this use of the offlap break might cause a misinterpretation of the ancient sea-level positions and the inferred relative sea-level changes.
2. both baselevels, the sea level and the wave/tide base, govern sedimentary accumulation in wave/tide dominated shelves and, consequently, two offlap breaks may coexist (beach edge and shoreface edge) in shallow-marine depositional profiles (Carter et al., 1991). In this setting, two seaward-clinobedded lithosomes, separated by an unconformity, may develop during relative still-stand or falls of the sea-level (Hill et al., 1998). In this case, the two stacked lithosomes could be misinterpreted as two different systems tracts, or sequences, and it could led to the construction of an uncorrect curve of sea-level changes.
Carter, R.M., Abbott, S.T., Fulthorpe, C.S., Haywick, D.W. and Henderson, R.A. (1991): Application of global sea-level and sequence-stratigraphic models in Southern Hemisphere Neogene strata from New Zealand. Sp. Publ. IAS, 12, 41-65.
Hernández-Molina, F.J., Fernández-Salas, L.M., Lobo, F., Somoza, L., Diaz-del-Rio, V. and Alverinho Dias, J.M. (2000): The infralittoral prograding wedge: a new large-scale prograda-tional sedimentary body in shallow marine environments. Geo-Marine Letters 20: 109-117.
Hill, P.R., Longuépée, H. and Roberge, M. (1998). Live from Canada: forced regression in action; deltaic shoreface sandbodies being formed. Abstracts, 15th International Association of Sedimentologists Congress, Alicante, 427-428.
Pomar, L. and Tropeano, M. (2001). The Calcarenite di Gravina Formation in Matera (southern Italy): new insights for coarse-grained, large-scale, cross-bedded bodies encased in offshore deposits. AAPG Bull. 85/4: 661-689.
Relative sea-level curve of the southern Baltic
Szymon Uscinowicz - Polish Geological Institute, Branch of Marine Geology, Poland, 80-328 Gdansk, st. Koscierska, Poland
The curve of relative sea level changes was determined basing on radiocarbon datings of 281 samples from 149 sampling sites. Following deposits were dated: peat (146 dates), fresh water gyttja (26 dates), marine mud (53 dates), lagoon mud (32 dates), marine shells (24 dates). The sedimentation environment of samples was confirmed by diatom or micro- and macro-fauna investigations. The dates were not calibrated and marine sea water reservoir effect were not taken into account. As additional, data about position of various forms related to coastal zone development and of reaches of erosional surfaces were used. The age of this forms was determined using the concept of depositional sequence seismo-stratigraphy as well as in several points was determined by means of radiocarbon and palynological dating.
The complex water level changes in the Southern Baltic since the last deglaciation to mid Boreal period were controlled by interactions between deglaciation dynamics, glaci-isostatic rebound of thresholds, erosion of thresholds and global sea level rise. Because of that a few high water stand and drainage events occurred. The 14C dates and sediment sequences indicate the range of transgressions and regressions during the Late Pleistocene and Early Holocene. The time of this events is determined only on the base of data from southern Sweden (e.g. Svensson 1991, Bjorck 1995) and Danish Straits (e.g. Winn et al., 1986; Bennike and Jensen, 1998).
During the initial stage (ca. 13 to 12.5 ka BP) the Baltic Sea was drained through the Oresund Strait (Bjorck, 1995). The water level was lower than present by about 30 m. It is determined by the position of the proximal parts of the fluvioglacial deltas on the Southern Middle Bank and the Vistula River delta front. The emerging threshold in the Oresund Strait forced the water level rise ca. 5 to10 m. In the period 11.2 to 11 ka BP, during the first drainage, water level decreased by about 25 m, i.e. to ca. -50 m below the present level. The extent of the retreat was determined basing on the erosional cutting of the top of till bed on the southern slope of the Bornholm Basin and Slupsk Trough. The second high stand of the Late Pleistocene Baltic Sea, ca. 10.3 ka BP (maximal extend of the Baltic Ice Lake) did not exceed the -25 m level, which is confirmed by dates of peat from the Slupsk Bank (Uscinowicz & Zachowicz, 1991-1994) and Southern Middle Bank.
The retreat of ice sheet (10.3 to 10.2 ka BP) opened the passage at Mt. Billingen (central Sweden) and final drainage of the Baltic Ice Lake occured. The water level did not decrease below –50 to 52 m. The sandy-muddy lagoonal deposits dated at 10.65 to 9 ka BP occur in the Gulf of Gdansk at a depth 55 to 58 m. below present sea level (Uscinowicz & Zachowicz, 1991-1994), and also erosional surface of the Baltic Ice Lake deposits in the Bornholm Basin occur at the similar depths.
The Preboreal transgression and small regression during the Early Boreal period are poorly
documented in the Polish part of the Baltic with forms or 14C dates. However, analysis of sequences observed in seismoacoustic profiles and of the position of dated peat samples on the Slupsk Bank allows to conclude that the maximum level of transgression did not exceed the –25 m level.
Since the mid Boreal, ca. 8.5 ka BP Baltic Sea has a permanent connection with the ocean through the Danish Strait (e.g. Winn et al., 1986; Bennike and Jensen, 1998). At that time only residual uplift occurred in the Southern Baltic area and sea level rise was related mainly to ocean eustatic changes. The sea level rise started from a ca. -28 m below present sea level, and during 1000 years rose to ca. -15 m. The period 7.5 to 5.5 ka BP is best documented thanks to numerous 14C dates of peat and lagoon deposit samples. During that period sea level rose from about -15 to ca. -5 m below present. Since about 5 ka BP the sea level rose very slowly (ca. 0.5 mm/y), and since ca. 3 ka BP was nearly the same as today.
More accurate reconstruction of the sea level changes in the Subboreal and Subatlantic periods on the basis of the known forms and dates is open to discussion. The existing on the coast deposits and forms may indicate small variations of sea level, or what is more probable, they may be the result of short term extreme events, e.g. extreme storms.
Benike, O. and Jensen J. B. (1998). Late- and postglacial shore level changes in the southwestern Baltic Sea. Bulletin of the Geological Society of Denmark 45: 27-38.
Björck, S. (1995). A review of the history of the Baltic Sea, 13-8 ka BP. Quaternary International 27: 19-40.
Svensson, N.O. (1991). Late Weichselian and Early Holocene shore displacement in the Central Baltic Sea. Quaternary International 9: 7-26.
Uscinowicz, S. And Zachowicz, J. (1991-1994). Geological Map of the Baltic Sea Bottom 1 : 200,000, sheets: Leba, Nexo, Gdansk, Elblag. Pa_stw. Inst. Geol. Warszawa.
Winn, K., Averdieck, F. R., Erlenkeuser, H. and Werner, F. (1986). Holocene sea level rise in the western Baltic and the question of isostatic subsidence. Meyniana 38: 61-80.
The final stage of the Holocene transgression in the Puck
Lagoon area, southern-Baltic Sea as observed from the Rzucewo Headland
Szymon Uscinowicz1 and Grazyna Miotk-Szpiganowicz2 - 1 Polish Geological Institute, Branch of Marine Geology, St. Koscierska 5, 80-328 Gdansk, Poland - 2 Gdansk University, Department of Geomorphology and Quaternary Geology, st. Dmowskiego 16a, 80-952 Gdansk, Poland
The Puck Lagoon is a small and shallow basin that is isolated from the Gulf of Gdansk open waters by a partly submerged barrier, so-called seagull barrier and Hel Peninsula. The Rzucewo headland is situated on the western coast of the lagoon, at the foothills of the morainic upland. The lagoon area is situated at 0 to 2 m depth. The Puck lagoon, as for the whole Baltic Sea is tideless, and sedimentary processes are dominated by wind, waves and current.
The geological structure of the Rzucewo Headland and its surroundings was examined with 11 boreholes as well as two seismoacoustic. Five peat samples, one sample of sand with organic matter, plant remains and Cardium sp. and two samples of Cardium sp. shells were dated by 14C method. Also the palynological analysis of 11 samples was done.
The land part of the Headland rises up to approximately 1 to 1.5 m above sea level. A much larger part of the Headland is underwater at a depth of 0 to 2m below sea level. The whole form of a half-moon shape is approximately 800 m wide in the north-south direction and 450 m in the east-west direction; its area is approximately 250,000 m2. An average thickness of the marine sands forming the Rzucewo Headland is approximately 1.5 to1.6 m., and 375,000 to 400,000 m3 volume. The marine sands are underlain by Late Glacial, Lower and Middle Holocene limnic and swampy sand, mud and locally peat. A radiocarbon age of the peat top varied from 5830 ± 45 yr BP (Gd-15209) to 5520 ± 70 yr BP (Gd-7698). The 14C age of the Cardium sp. shells in the base of marine sand is 3,435 ± 30 yr BP (GdA-169) and 3,560 ± 35 yr BP (GdA-171), (Fig.1).
The study results of the Rzucewo Headland area indicate that the area until the end of the Atlantic period has been developing under the land conditions. At the beginning of the Subboreal period in the area the land conditions still prevailed and the transformation of the freshwater lake into the brackish/marine Puck Lagoon began not earlier than 5,000 to 4,500 years ago. At the beginning of the Subboreal period change of the species composition of the forest community was related to climatic-edaphic conditions. The trees of higher habitat requirements such as lime (Tilia) and elm (Ulmus) nearly totally disappeared and were replaced by hornbeam (Carpinus) and beech (Fagus) (Miotk-Szpiganowicz, 1997). At the same time, ccurrence of pollen grains of the ruderal plants such as the goose-foot (Chenopodiaceae), the motherwort (Artemisia), sorrels (Rumex) and specially more frequent the plantain (Plantago lanceolata) indicate increased human activity in the area. The results of the pollen analysis and the data of the sea level are well correlated with archaeological data (Król, 1997) that determine the beginnings of the seal hunters settlement in Rzucewo at approximately 4,400 years ago.
In the Subboreal period, cliffs on the slopes of the Morainic Uplands started to develop and the accumulation of sands in the Rzucewo Headland begun. In the last approximately 4,000 years, an average growth of the Rzucewo Headland was around 100 m3/year. The geological data (core profiles) indicate that the Rzucewo Headland development occurred under conditions of the mildly ceasing transgression. No facts pointing to either transgression phases or existing of periodical regressions were found. The location and range of the marine (lagoonal) sediments documented in the Rzucewo Headland area indicate that the water level of Puck Lagoon was not higher than the recent one and that the changes of the shoreline position are related to the accumulative processes, which happened in the Upper Holocene at the very slow rise of the water level. This fact is important to the palaeogeographical interpretation, particularly in view of the recent vertical movements of the Earth crust in the Puck Lagoon area being close to 0 or only slightly negative (up to –0.5 mm/year) (Wyrzykowski, 1985). In the last 5,500 years the curve of the relative water level changes of the Southern Baltic (Uscinowicz, 2000) and Puck Lagoon is in good agreement with the curves of the eustatic changes of the ocean (e.g. Mörner, 1976; Blanchon and Shaw, 1995) (Fig. 2). This points to a small range of the vertical movements of the Earth crust and local eustatic changes in the Subboreal and Subatlantic periods in the area.
Blanchon, P. and Shaw, J. (1995). Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and ice-sheet collapse. Geology 23/1: 4-8.
Krol, D. (1997). Excerpts from Archaeological Research at Rzucewo, Puck Region. In: D. Król (ed.), The Built Environment of Coast Areas During the Stone Age. The Baltic Sea-Coast Landscapes Seminar. Session No.1, Gdansk, pp. 135-150
Miotk-Szpiganowicz, G. (1997). Results of palynological investigations in the Rzucewo area. In: Król, D. (ed.), The Built Environment of Coast Areas During the Stone Age. The Baltic Sea-Coast Landscape Seminar, Session 1, Gdansk, pp. 153-162.
Mörner, N.-A. (1976). Eustatic changes during the last 8,000 years in view of radiocarbon calibration and new information from the Kattegatt region and other north-western European coastal areas.
Uscinowicz, S. (2000). Late Glacial and Holocene of the Southern Baltic shoreline displacement. Geoindicators, Symposium and Field Meeting, Abstracts and Guide-Book of the Excursion, GSA & Polish Geological Institute, Gdansk, pp. 36-37.
Wyrzykowski, T. (1985). Map of the recent vertical movements of the surface of the Earth crust on the territory of Poland 1 : 2,500,000. Instytut Geodezji i Kartografii, Warszawa.
Submerged features related to the LGM in the Argentine
continental shelf: the present knowledge
Roberto A. Violante - Division of Marine Geology and Geophysics, Argentine Navy Hydrographic Office, Buenos Aires, Argentina
The Argentine continental shelf covers about 1,000,000 km2, adjacent to a coastline longer than 4,000 km, extended in a north-to-south direction from 35 to 55°S. Its width ranges between 170 to 850 km, its slope is gentler than 1 : 4,000 and the shelf edge occurs at a depth of 110 to 165 m. Related to a passive margin and modeled by Late Quaternary glacial-eustatic events, its present configuration was attained during the LGM subaerial exposure and the subsequent Holocene transgression.
Although research activities begun in the fifties and many geological, geophysical and morphological aspects are well known, studies were not specifically focused in defining the LGM, and only few of them obtained results concerning the position of ancient shorelines and features correlated with the last lowermost sea-level. In the last two decades, however, more detailed underwater seismic and geological studies oriented to interpretate Late Quaternary sedimentary sequences in the northernmost inner shelf and adjacent coastal plains were carried out, which contributed to a better understanding of geological processes and events occurring during the Late Pleistocene-Holocene. After compiling most of the present knowledge based on information coming from papers by many authors (which are not cited here due to lack of space), it was possible to get a broad picture of features and processes related to the LGM.
The most recent studies based on radiocarbon dates from the Argentine continental shelf reveal that the lowest sea-level at the LGM reached ca. 150 m below present near 16,690 corrected yr BP, although is still much discussion on the timing of the event; the eustatic component of the subsequent deglacial rising of sea level (after substracting tectonic and hydroisostatic events) was ca.105 m (what is considered as a minimum paleo sea-level). The rate of the relative sea-level rise can be inferred as being ca. 8.9 m/1,000 years at least up to about 8,000 yr BP.
At the time when sea level was at its minimum during the LGM, the present shelf was an extensive subaerial plain with fluvial systems overimposed. With the rising of sea level, it was flooded and shaped depending on the interaction among relative sea level fluctuation, climatic conditions, fluvial - glacial and marine processes, coastal and submarine configuration, coastal dynamics, sediment supply, erosion-deposition rate and neotectonic factors. As a result, diverse geomorphological features were formed. Although more detailed studies are still lacking, its characteristics make possible to regionally differentiate three broad regions.
The northern region, around La Plata river (referred here as Río de la Plata), is characterized by the influence of a huge fluvial-estuarine environment, which was during the late Cenozoic (as it is today) the most important route for sediment transport to the sea. In fact, in this region fluvial activity dominated over marine processes. During the LGM, the river was narrower than today with a large capability to transport sediments, which run aside the Uruguayan coast flowing into the sea in front of the present coast of Río Grande do Sul (southern Brazil), where the significative sediment supply was able to built an extense delta complex. Coastal areas around the ancient river mouth showed beaches, tidal flats and coastal lagoons. With the rising of sea level, the sea invaded the river valley and its marginal areas giving origin to an estuarine environment which constitutes the present Río de la Plata.
The central region of the shelf, which comprises the areas adjacent to eastern Pampas and most of Patagonia, was during the LGM a gentle plain bordered by coastal plains. Marine processes dominated here, fluvial processes were of minor importance and there was no glacial influence. Coastal environments were represented by sandy barriers-muddy lagoons systems in the northern areas and gravelly to coarse sandy beaches to the south, whereas muddy estuaries and tidal flats characterized the enclosed areas. Deltas and associated coastal environments formed at the mouth of the larger rivers, and submarine canyons dynamic was important in deeper areas today located offshore the shelf edge. During the subsequent transgressive event coastal systems moved upslope as sea level rose. Coastal retreating produced a ravinement surface at the top of the pre-transgressive substrate, which was progressively covered by sediments reworked from the migrating littoral environments giving origin to a sandy relict mantle which extends over most of the shelf, overlying Plio-Pleistocene continental and marine deposits in areas adjacent to the Pampas and middle to late Tertiary sediments in areas adjacent to Patagonia.
The southern region, in Tierra del Fuego and the tip of Patagonia, behaved during the LGM in a very different way than northern regions did. The narrow continent was here, at that time, almost completely covered by ice, in such a way that glaciers reached positions in the vicinity of the present coastal areas and even seaward. Resulting deposits associated with glacial-fluvial processes were later partially submerged during the sea level rise and appear today in the shelf as well as in the bottom of the drowned valleys as Magallanes and Beagle straits.
As a conclusion, LGM and early transgressive submarine deposits on the Argentine continental shelf comprise distinctive features such as deltas, beaches, coastal barriers and lagoons, estuaries, tidal flats, relict sandy blankets, glacial-fluvial deposits and submarine canyons, depending on the local influence of different geological events and processes. All those features can be recognized after palaeoenvironmental studies (based on sequence-stratigraphy) as they are preserved in the geological record as falling stage, lowstand and transgressive systems tracts. However, little is still known on the subject. Future studies need first to be carried out on a regional basis to determine broad general aspects of the shelf and the processes related to LGM, and then, to focus more detailed local surveys in specific key areas where the most significative geological-morphological features related to the last lowest sea-level stand are present.
A preliminary study on the sea-reach courses of the Huanghe
and Changjiang rivers during the Last Glacial Maximum
Dongxing Xia - First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China
The Huanghe and Changjiang Rivers are the main suppliers of sediment for the Eastern China Seas. Traditionally, it was regarded that the Huanghe River flowed via the Bohai Sea, the Yellow Sea and the East China Sea, and finally entered the north part of the Okinawa Trough during the maximum of the last ice age as the sea level was lowered; while the Changjiang River flowed in a course extending southeastward from the present Changjiang River mouth, and entered the middle part of the Okinawa Trough.
Having made a detailed analysis on the climatic environment of the maximum of the last ice age, the Quaternary stratigraphic patterns of the North China Plain and the seismic stratigraphic profiles in the Eastern China Sea shelves, the author regards that during the maximum of the last ice age, the climatic environment in the North China Region was quite arid, the mean annual rainfall in the upper- and middle-reach areas of the Huanghe River was only 200mm, that in the lower-reach area of the river was about 300mm. Thus it was impossible for the Huanghe River to catch enough running water to flow through the exposed shelf plain to enter the lowered sea, and hence the river was disintegrated and stopped running at about 21ka~13ka B.P. The deltaic deposition of the Huanghe River during the maximum of the last ice age was not found in the shelf area of the Bohai and Yellow Seas, while eolian sand deposits were extensively developed in those areas. The paleo-channels of the Changjiang River and the relevant sedimentary system formed during the maximum stage of the last ice age were not found in the shelf area of the East China Sea to the south of 328N. So the Changjiang River did not flow eastwards from the present Changjiang River mouth to enter the middle part of the Okinawa Trough, while it flowed eastwards from the middle part of the North Jiangsu Plain, along the boundary zone of the Yellow Sea and the East China Sea, and entered the north part of the Okinawa Trough. The so-called paleo-channel to the southeast of the present Changjiang River mouth is actually a modern tidal channel, because the tidal current velocity measured currently can reach 120cm/s.
Evolution of Subei (northern Jiangsu Province, China) coastal
plain in the Holocene and the contributions of Changjiang and Huanghe sediments
Shou-Ye Yang,1, 2 Cong-Xian Li,1 Hoi-Soo Jung2, and Hee-Jun Lee2 - 1 Department of Marine Geology and Geophysics, Laboratory of Marine Geology, Tongji University, 1239 Siping Road, Shanghai 200092, China - 2 Marine Geology and Geophysics Division, Korea Ocean Research and Development Institute, Ansan, P.O. Box 29, Seoul 425-600, Korea
The Subei coastal plain, located between the modern Changjiang and old Huanghe Rivers, occupies about 3,000 km2 in area (32º30´N-33º50´N). The Huanghe entered the Yellow Sea nearby during the period of 1128-1855 AD and delivered huge amounts of sediments to the coastal area and the Yellow Sea (Ren and Shi, 1986). This area is also considerably influenced by the development of the Changjiang Delta (Li et al., 2001). Therefore, the Subei coastal plain can be a typical area for studying the land-ocean interaction and characterizing the late-Quaternary paleoenvironmental changes of East China.
The thickness of postglacial strata ranges from less than 10 m to more than 30 m, decreasing northward and westward. The sedimentary sequence is generally composed of basal fluviatile facies, coastal facies, littoral-neritic facies and the topmost littoral- tidal flat facies. C14 dating revealed that the coastal facies was formed during the early postglacial transgession, while the littoral-neritic facies was a regressive sequence with typical tidal sand ridges having formed after maximum flooding (about 5,000-7,000 a BP)(Li et al., 2001). The littoral-tidal flat facies was formed in nearshore environments with the upper part dominated by tidal flat deposition.
Geochemical and mineral compositions of five drill cores from Subei coastal plain were determined and compared to those of Changjiang and Huanghe sediments. A clear difference in the geochemical compositions between Changjiang and Huanghe sediments can be used to identify both river sediments in coastal area and marginal seas (Yang and Li, 2000). Most of elemental compositions of the core sediments display obvious trends, varying between those of Changjiang and Huanghe sediments. The provenance index (PI) for core sediments was calculated using a two-end-members mixing model, based on nine stable elements (Ti, Al, Co, Sc, Ni, Cr, Nb, Be, and V). While varying between the end values of the Changjiang and Huanghe sediments, the PI values show a general trend of increasing from northern cores to southern cores, and, for the central cores, increasing downcore. This indicates alternating influences of the Changjiang and Huanghe on the coastal plain through Holocene time.
During the early postglacial transgression (12 to 7 ka BP), the Changjiang influenced this area extensively and contributed much sediment for the development of the coastal plain, whereas the Huanghe entered the Bohai Sea and exerted negligible influence on this area. During the middle Holocene (7 to 3 ka BP), the Huanghe had occasionally flowed across the northern Jiangsu coastal area (Ren and Shi, 1986; Alexander et al., 1991), and the Changjiang and the Huanghe together supplied considerable amounts of sediment to the coastal plain. However, during the late Holocene (3 ka BP to present), the frequent switches of the Huanghe into the coastal plain resulted in the dominance of the Huanghe sediments therein, especially during the period of 1128-1855 AD, the Huanghe supplied huge sediments to the coastal area and the coastline prograded rapidly during this period. At the present time, the Jiangsu coastal plain receives negligible amounts of modern Changjiang and Huanghe sediments because the Changjiang and Huanghe entered the East China Sea and Bohai Sea respectively, and most of the river sediments are trapped in the estuarine areas. Therefore, the abandoned Huanghe Delta has been undergoing strong erosion, and the eroded sediments are mostly transported southward and deposited in the southern coastal area and even further to the outer shelf (Ren and Shi, 1986).
Alexander, C.R., DeMaster, D.J. and Nittrouer, C.A. (1991). Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea. Marine Geology 98: 51-72.
Li, C. X., Zhang, J. Q., Fan, D. D. and Deng, B. (2001). Holocene regression and the tidal radial sand ridge system formation in the Jiangsu coastal zone, east China. Marine Geology 173: 97-120.
Ren, M.E. and Shi, Y.L. (1986). Sediment discharge of the Yellow River (China) and its effect on the sedimetation of the Bohai and the Yellow Sea. Continental Shelf Research 6: 785-810.
Yang, S.Y. and Li, C.X. (2000). Elemental compositions in the sediments of the Changjiang and the Huanghe Rivers and their tracing implication. Progress in Natural Science 10: 612-618.
Recognition of post-glacial and pre-post-glacial sediments
on continental shelves:lessons learnt from the Hong Kong SAR, China
W.W.-S. Yim - Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
The recognition of post-glacial and pre-post-glacial sediments on continental shelves is needed for the reconstruction of sea-level changes. Based mainly on lessons learnt from Hong Kong, features useful for distinguishing between them include palaeontological properties, sedimentological properties, mineralogical properties, chemical properties, engineering properties and absolute dating. In this paper, selected features will be described together with an evaluation on some of the dating methods. No good evidence has been found to distinguish between the sub-aerial exposure of the present-day continental shelf during the Younger Dryas episode and the Last Glacial Maximum. This will continue to be an important area for future research.
Review of results of International Geological Correlation
Programme project no. 396 ‘Continental shelves in the Quaternary’
W.W.-S. Yim - Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
IGCP 396, a 5-year project from 1996 to 2000 was sponsored by UNESCO and the International Union of Geological Sciences. The project was aimed at the study and interpretation of Quaternary sequences on continental shelves to permit global correlation of sea-level and climatic changes while at the same time attempts to identify beneficial uses for humankind. Conferences aimed mainly at the promotion of scientific exchanges between over 400 participants from 43 countries were in Sydney (Australia 1996), Durham (United Kingdom 1997), Goa (India 1998), Cape Town (South Africa 1999) and Rio de Janeiro (Brazil 2000). Highlights of results of the project include:
1. Development of new drilling technology
2. Recognition of the importance of relic sediments
3. Validation of Quaternary ages using a range of dating methods
4. Studies of sea-level changes and sequence stratigraphy
5. Recognition of a long uplifted Pleistocene shelf sequence in New Zealand
6. Recognition of ca. 0.5 Ma sequences in the South China Sea inner shelf
7. Identification of palaeosols formed during low sea-level stands
8. Land-sea correlation
9. High-resolution palaeoclimatic records from corals
10. Discoveries in marine archaeology
11. Improvement in understanding of engineering properties of shelf sediments
12. Recognition of the importance of shelves in the global carbon cycle
13. Recognition of the importance of shelves for living and non-living resources
IGCP 396 was a highly ambitious project covering a wide spectrum of topics of important societal relevance. The project is a starting point of an international collaborative effort to improve our understanding of continental shelves in the Quaternary. More funding is needed at the national, regional and international levels for its successor project IGCP 464 Continental shelves during the last glacial cycle: knowledge and applications.
Human activity of the Vistula delta plain and Vistula lagoon
shoreline displacement during the Holocene
Joanna Zachowicz - Polish Geological Institute, Branch of Marine Geology, 80-328 Gdansk, st. Koscierska 5, Poland
The Vistula Delta Plain is situated in the north of Poland on the southern coast of the Baltic Sea. It covers an area of ca. 1800 km2 and developed inside a lagoon cut off from the Gulf of Gdansk by the narrow Vistula Barrier. The delta stretches at an altitude ranging from about 10 m a.s.l. where the Vistula branches into two main streams, to –1,8 m b.s.l. in its north-eastern part. The relief of the top of Pleistocene sands and locally tills in the Vistula Delta Plain is erosive. The lowest erosive surface lies at more than 30 m. below sea level, while the uppermost part reaching a few metres above the present surface of the delta plain (Mojski, 1995). The Holocene deltaic sediments consist two lithostratigraphic units. The lower unit consist of medium and fine-grained sands with inserts of oxbow deposits. The younger part of the lower unit consist of muds and peats formed between 8 and 6.3 ka B.P. The lower unit is generally from several to about a dozen metres thick. The upper unit is similar to the lower one. In its lower part large amounts of channel facies fine sand prevail with numerous inserts of phytogenic material. The younger part of upper unit is dominated by muds. In the north and east of the area, deltaic deposits of the upper unit are replaced by marine (lagoonal) sediments. The lagoonal deposits of the Vistula River Delta are younger then 6.3 ka BP (Mojski, 1995).
The history of the Vistula Delta Plain and human activity in this area is presented on the basis of pollen analysis of three cores from Vistula Lagoon (Zachowicz 1985) and Lake Druzno (Zachowicz et al., 1982) as well as the location of different age archaeological sites.
Conceptual model of tidal sand ridge development since
the last deglaciation in East China continental shelf
Kelin Zhuang1 and Zhenxia Liu2 - 1 Qingdao Institute of Marine Geology, CGS, Qingdao, 266071, China - 2 The First Institute of Oceanography, SOA, Qingdao, 266061, China
A conceptual model of tidal sand ridge development is made possible based on a series of comprehensive studies on Bohai Sea, Yellow Sea and East China Sea. The development of tidal sand ridges correlates well with transgressions since the last deglaciation. All the tidal sand ridges in East China Continental Shelf, which include Bohai Sea, Yellow Sea and East China Sea, can take into 6 groups which correspond well with sea level rises in this region. The tidal sand ridges were constructed simultaneously in Bohai Sea, Yellow Sea and East China Sea in the same early period of sea level rise. With the transgression after the last deglaciation, tidal sand ridges developed first in East China Sea. After seawater reached Yellow Sea, they developed simultaneously in Yellow Sea and East China Sea. Finally when seawater reached Bohai Sea, they develop contemporarily in Bohai Sea and Yellow Sea, while tidal sand ridges in the East China Sea became moribund. Each period of tidal sand ridge development lasted for about 2 ka.
Postglacial sea-level rise and palaeo-shoreline movement
along the northern continental shelf of the South China Sea
Y. Zong,1 Z. Huang and W. Zhang2 - 1 Department of Geography, University of Durham, Durham, UK - 2 Guangzhou Institute of Geography, Yellow Flower Hill, Guangzhou, China
Dramatic changes in climate have taken place since the Last Glacial Maximum (LGM). As a consequence, land-based ice sheets retreated rapidly, and large amount of meltwater was released into the world's oceans, causing rapid eustatic sea-level rise. However, the increase in global temperature since the LGM has not been steady. There were periods of rapid increase of temperature, resulting in pulses of increased meltwater discharge into the oceans (e.g. Fairbanks, 1989). Between these time periods of rapid eustatic sea-level rise are stages of RSL slow down, during which shorelines may have developed with recognisable coastal materials left for us to discover.
According to the varied interplay between eustatic and glacio- and hydro-isostatic processes, the history of relative sea-level (RSL) movements has been drastically different between so-called near, intermediate and far field sites (e.g. Peltier, 1998). The South China Sea is far from the former ice sheet margins. As a far-field site, the hydro-isostatic effect outweighs the glacio-isostatic signal, particularly where an extensive continental shelf exists. Across such a continental shoreline, the hydro-isostatic processes have two effects, oceanic submergence of the continental shelf due to hydro-isostatic loading and ocean margin emergence or crustal tilting of current shoreline (Clark et al., 1978). Furthermore, tectonic movements may also be important in far-field sites, because the combined isostatic effects are usually small. Therefore, all these factors have to be taken into consideration when RSL and shorelines are
Recent studies have accumulated evidence of submerged deltas and shorelines on the northern continental shelf of the South China Sea (e.g. Huang et al., 1995a; Zhao, 1996). With regard to the LGM shoreline, however, both morphological and sedimentary evidence are insufficient, leading to various opinions on the position of the LGM shoreline (Huang et al., 1995b). Similarly, various postglacial shorelines have been proposed, but not substantiated. This paper re-examines all lines of evidence, with consideration of the complex eustatic, hydro-isostatic and tectonic effects and cross-reference to evidence from nearby regions, such as the continental shelf of East China Sea and the north Australian continental shelf. For mid-Holocene and Late Holocene shorelines, evidence are drawn from the Pearl River Delta, Han river Delta and the south coast of China.
The LGM shoreline has not been identified although there are various proposals suggesting its position raging from -60 to –160 m (Huang et al., 1995b). This leads to the question of whether or not RSL during the c. 80,000 years of Late Glacial was stable, and for such a long period what palaeo-environmental conditions were there.
The 13,000 to 12,000 yr BP shoreline is much easier to identify, since it occurs between the front of the submerged delta (Huang et al., 1995a) and sand sheet belt. As this shoreline formed during the initial period of postglacial RSL rise, its position varies at a range of depths from -130 to -120 m.
Meltwater pulse Ia caused a rapid rise in RSL which reached -50 m by ca. 11,000 yr BP.
The 11,000 to 10,000 yr BP shoreline formed during the slow down of RSL rise after the meltwater pulse Ia. At about –50 m, coastal deposits and landforms are plentiful (Zhao, 1996).
Meltwater pulse Ib is much smaller in magnitude in comparison with Ia. It helped RSL to rise to ca. -20 m.
The 9,000 to 8,000 yr BP shoreline exists because intertidal deposits including shells and other morphological evidence are found from ca. -20 m. However, along the front of the present-day deltas, this shoreline is much more difficult to locate, since part of the deltas was possibly inundated.
Mid-Holocene highstand and the 6,000 yr BP shoreline are both well documented. The height of RSL was very close to the present sea level. The shoreline reached its maximum landward positions in both the Pearl River Delta and the Han River Delta.
There is a possible Late Holocene highstand, and the 2,500 yr BP shoreline re-advanced inland.
Clark J.A, Farrell W.E. and Peltier, W.R. (1978). Global changes in postglacial sea level: a numerical calculation. Quaternary Research 9: 265-287.
Fairbanks, R.G. (1989). A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature 342: 637-642.
Huang Z. Zhang W. and Cai F. (1995a). The submerged Pearl River Delta. Acta Geographica Sinica 50: 206-213.
Huang, Z., Zhang, W., Cai, F. and Xu, Q. (1995b). On the lowest sea level during the culmination of the latest Glacial period in south China. Acta Geographica Sinica 50: 385-393.
Peltier, W.R. (1998). Postglacial variations in the level of the sea: implications for climate dynamics and solid-earth geophysics. Reviews of Geophysics 36: 603-689.
Zhao, X. ed. (1996). China Sea-level Change. Shengdong Science and Technology Press, pp. 464 (in Chinese).
|Application of magnetic properties for studying modern seabed sediments contaminated by shipping activity in Hong Kong Harbour - Lung Chan and Wyss Yim|
|Growth history of coral reefs since the Last Glacial Maximum in the western margin of Australia - Lindsay Collins|
|Human activity of the Vistula delta plain and Vistula lagoon shoreline displacement during the Holocene - Joanna Zachowicz|
|Relative sea-level curve of the southern Baltic - Szymon Uscinowicz|
|Stratigraphy and sea-level history of the late Pleistocene Sunda Shelf Till Hanebuth, Karl Statteger and Yoshiki Saito|
|The final stage of the Holocene transgression in the Puck Lagoon area, southern Baltic Sea as observed from the Rzucewo Headland case study -Szymon Uscinowicz and Grazyna Miotk-Szpiganowic|
|The recent outlet area of Vistula River - Szymon Uscinowicz , Dorota Koszka-Maron, Joanna Zachowicz , Marek Graniczny|
Seismic and sedimentological characters of a 5th-order depositional sequence
formed during the last glacio-eustatic cycle
E. Martorelli, Francesco Chiocci and G. Ercilla
|Ferry leaving to Lantau Island||
|Transfer to Penny's Bay (full resolution)||
Fishing at the ferry station
|Hopper at work (full resolution)||
|Rainbow filling (full resolution)||
Geotechnical drilling vessel
Earth Water Metal Fire Wood
Two hopper reclaiming at once
Project illustrated by chief engeener
French sea horse
Polish sea horse
Monster honorary advisor
Italian sea horse***
Polish sea horsess
Tired people return home
Argentina sea horsess
Brazilian sea horse
Who is the true chinese waiter?
Australian sea horse
Italian sea horse
(le riz coulè)